I bought a lot of burlap type material in a roll.
I cut the material to where it was long enough and wide enough to allow me to us an electric upholstery stapler to fasten it first along one long side along the inside edge, then pulling it slightly taut stapled it to the other long side on the inside edge starting at the middle and pulling gently toward the ends to pull any wrinkles out.
Then a did one short end and did some corner tucking like a Christmas present stapling all to th inside, then went to the other end pulling gently to make the material taut then stapling the end material to the inside.
Once that was done I forced the mineral wool inside to stretch the frame out.
if you are going to be moving the panels around much you can wrap the mineral wool in a light fabric to keep it from shedding before putting it in the frame, otherwise if it is going to be fixed, it wont matter.