Been nerding out reading lots of GS acoustics threads for awhile. Many of the trap designs are ingenious. Inexpensive in materials, easy to assemble, good-looking, etc.
Desiring to use cheap Roxul Safe'n'Sound, which is somewhat floppy and fragile, I wanted a trap rigid enough to protect the rockwool, but as light as possible. Neither over-built nor under-built, and preferably using scraps already in my wood rack. Get rid of a bunch of pieces too small for most projects but too big to discard, "Gee I might need a scrap that size some day."
The first corner-straddle trap, internal dimension 9" deep, 70 1/2" tall, 22 1/2" wide. External dimensions 10" deep, 72" tall, 23 1/2" wide. Except for a bottom plate of 3/4" plywood, all made of 1 1/2" wide sticks cut out of half-inch plywood on the table saw. Assembled mostly with titebond III glue and finish nailer, 1.25" 18 gauge nails. Skinny little nails.
I usually fit pieces together with the glue, clamp temporarily if necessary, and waste enough airgun nails that it can't move til the glue dries, then remove clamps and move on. No glue-up then wait for hours.
It is open on front, back, both sides and top. Burlap fabric. 7" opening on sides and top, about 20" opening on front and rear. I didn't like making the vertical ribs 1 1/2" wide, but half-inch plywood strips are bendy, and having never made this, didn't want it too wobbly, though fixing two strips at right angle on each corner increases the stiffness.
The 1" horizontal braces, was worried that they might not glue in firmly without extra support, or taking the time to cut splint inserts, so added the little triangle reinforcements on each side of the horizontal braces "just in case".
The batt size is 47" X 15" X 3". So the box is filled with three layers, 1.5 batts tall and 1.5 batts wide. First layer, on bottom, 1 full batt beside a half batt cut vertically. On top, a half batt cut horizontally, aside a quarter batt cut both horizontally and vertically. Electric bread knife cuts the stuff trivially easy.
The second layer of rockwool puts the big pieces on top, swapped left-to-right, so that seams don't go all the way thru front to back. Then the third layer has the same layout as the first layer. Was thinking perhaps the overlapping would also help support the insulation in the box. Assembly sequence--
Made the open top out of sticks, cut the solid bottom piece, then attached to the top&bottom, vertical supports for front and sides. Attached the horz supports.
Then stapled in one big piece of burlap for front and both sides, and a small piece of burlap for the top opening. Discovered that Titebond III glues burlap to wood GREAT! After stapling in the fabric, ran a bead of the glue down each side, then use finger to dab/spread the glue down thru the fabric along the staple line. As the burlap wets with glue it lays right against the wood and glues to the wood firm after an hour drytime.
Burlap looks real likely to unravel, so I didn't trust mere occasional staples to keep it from unraveling around the edges over time. With every horz and vert strand glued firm to the edge struts, seems that as long as the fabric strands don't start breaking in the middle, it ought to stay in place fairly well over time.
After installing the rockwool, I stapled in the back burlap. Ran beads of glue over the fabric and smoothed the glue in, then nailed on the back sticks. So that the back burlap is hard-glued in a sandwich between the side sticks and back sticks.
One hazard with this method-- Because the unit is somewhat floppy until all pieces have been assembled, it is easy to mistakenly pull the back burlap too tight while stapling, and bow-in the rear sides. Which will somewhat fix itself when the back sticks are nailed on, but better to avoid the issue by not pulling the fabric so tight.
Ran masking tape around all exterior fabric edges and used router 1/4" round-off bit to round all edges that had not been previously rounded off. The tape helps keep sawdust out of the fabric, and obviously makes the box easier to paint without painting the fabric.
Filled a zillion tiny nail holes with elmers white wood filler. Final sanding.
One of my favorite paints, perhaps usa-only, is a Glidden water-based primer called Gripper. Gripper will stick to anything, is tough as nails, very difficult to scratch or even sand off, and dries incredibly fast. Smells kinda nasty as water-based paints go, but not an intolerably bad smell, and the smell doesn't linger after dry.
Good-- The box is fairly light, easy to pick up and move. It is fairly rigid. Easily able to support the rockwool and itself. Inexpensive, about $20 of rockwool, less than a half sheet of half-inch plywood, plus burlap, staples, nailgun nails, glue, filler, paint.
Indifferent-- The box is does have some torsion, twisting wobble if you pick it up in such a way that the box is twisted along the long dimension. But it is not necessary to twist the box to pick it up and move it. Torsion problem most obvious getting it off the workbench and into the office, with just one person (me). Not a problem for a box that is only moved occasionally, but would need to be stronger to go on the road.
Not so good-- It took too long to assemble. Building the front and sides, then stapling the fabric inside, it was annoying to stretch and staple the fabric down inside the box. A little crowded stapling inside the almost-complete box.
So the next trap was made the same size and overall appearance, but with modified construction to see if it could be made faster and a little more rigid, with bigger openings and smaller struts.