I get it now. Look at it again though. That is a 2" thick piece of absorbent. The outer and inner frame are outside of the material, so for a 2" thick trap, the spacers are 2" thick. To use this design, I would just follow it. Making the frame smaller and snugging is no easier than notching the corners of some insulation. Having the insulation flush with the wood frame on the face... I see no benefit to this. Depending on the insulation you use (rockwool for example) the surface can be quite irregular. Also, this design strikes me as having limitations to how deep you will be able to go. That stick frame will only allow so much weight, I'd think.
This design seems rather labor intensive to me, and it doesn't allow for an easy air gap against the wall... there is also no fabric on the back side. Are you doing corner traps or wall traps?
Here's my favorite wall panel. Frame is a 1x2". This creates a 2" airgap to the wall. To the front of that frame you attach a very cheap fabric (tan in this picture. This fabric is sandwiched between the frame and your insulation...in this case 2" thick. You then wrap that whole package in your finish fabric. Assembly is easy. Lay down the finish fabric (good side down). Lay your insulation on top of that. Then lay the frame down, with crap fabric touching the insulation. Pull the good fabric around the edges and staple it to the frame. Nice gift wrapping style corners. THe exposed frame on the rear makes hanging a breeze.
my 2 cents.