From what I can find out, the Guitar Mate Reverb came with a 12inch,8 ohm speaker, they were listed as 20 watts, probably about 14-15 watts , a cool little amp, I'd like to find one myself, do you plan to just build a speaker cab, or are you going to re-combo-ize the amp? Either way 3/4inch Apple ply or Birch ply, is the way to go, use cabinet grade plywood, a half sheet should be plenty (4ftx4ft), if you are going to build a closed back cab, make sure you fit the back and baffle board tightly, as close to airtight as possible and I wish I was in your shoes, I love doing stuff like this, if you want more info, just do a search on this site, Building Speaker Cabinets, and you should hit the motherlode, this is Lordathestrings, Favorite subject of all time! LOL, ROFL, chuckle, chuckle, chuckle................