I'm planning to mount some speaker boxes in my front kick panels and in some rear side panels I'm planning to make. All will be mounted in sheet steel and I'd like to make some decent sounding speaker boxes to go behind them. Anyone have any experience with materials that will work well for this? I know that MDF makes for great sub boxes, but I don't have room for something bulky.

I'd build a metal box with the inside of the box covered in dynamat or similar then I'd fill the box with poly fill(pillow stuffing). Also, I have a large hole saw you can use if you want. I used it to make perfect holes in my kick panels when I put speakers there. I'll have to measure the size.

Just what Jason said. Gotta line em with dynamat and fill the boxes. otherwise it will have a "tin" sound to them.