I had a old altec speaker cab with 4 12's and every one ot the speakers had a ripped cone except one.
I found a half sheet of plywood (1/4 inch) in my shop and took a jigsaw and cut out a hole to mount the speaker in on a pretty much square piece, and 5 pieces for the sides, top/bottom, and the back.
I nailed the thing together and caulked all the seems with silicone.
I just pretty much eyeballed it because I wanted something very quick to try the valve jr amp out on.
I guess I lucked out because it sounds better than anyu other speaker I have tried it on.