Here is a suggestion on how to construct the frame out of wood and leave a lot of the sides, top and bottom exposed. You still have a sturdy frame for mounting. You can mount spacers on the rear to get them say 2" off of the wall.
1) cut a piece of fabric about 64" X 42"
2) lay the fabric on a flat surface
3) lay the frame face down on the cloth
4) using a staple gun, staple one of the long edges of the cloth onto the rear of the frame
5) while pulling and stretching the other long side staple it on the rear of the frame
6) then do the same for the top and bottom, you will have to fold the corners
7) lift up the frame and make sure the cloth is pulled tightly with no wrinkles
8) put the frame back on the floor face down and put the mineral wool into the frame
9) if you want to (which I do) get some 1/2" air filter material meant for paint booths from an air filter supplier (they are in every town). Put the filter material into the frame so that it will cover all exposed mineral wool. Put in thge mineral wool and then cover the rear of the mineral wool with the filter material.
10) You can stretch some material across the rear of the panel for looks if you wish to.
Note: If you want to use the best fabric available both for looks, durability, streatchability and fire rating, use Guilford of Maine F701 -2100 available in many colors.
I enclosed a pdf with a suggested method of building the frame