I think I agree with blaser... 80L is plenty big enough. I had no idea how much space you had so I just picked a number. With the 80L box and a 25Hz tune it looks nice. The extra 50L from the 20" cube makes very little difference. As for the box; the most common material is 3/4" MDF because it's super cheap and very dense. If you wanted you could get the diy plans from
www.rythmikaudio.com and just cut the appropriate sized hole for your driver.