Thanks guys!
I used both glue and screws. I figured if I just used glue, it would let loose after a while, and if I just used screws, they would end up stripping out after a while, so I doubled up just as extra assurance. I just ripped the wood down to the sizes I wanted, and cut some solid 3/4 x 3/4 pieces for each corner, glued them in, and put screws in from the top and sides. It's reletively easy to build, if you have some basic math skills, and can use a tape measure, a saw, and a drill. I ended up running out of carpet, so I couldn't cover the entire inside like I had originally planned, but that's always somthing that can be done later.
For reference if anyone wants to build one:
I used 1/2" plywood for the bottom. Everything else is 3/8"
Top and bottom pieces: 27" x 12"
Shelf piece: 27" x 10" - This will give some clearance for the vent on top of the amp. Or you could make it 12" wide, and cut out the part over the vent if you want.
Sides: 19 3/4" - This will give you enough room for the head, 4 rack spaces (1.75" x 4 = 7) and allows for 1/4" clearance on top and bottom of the rack. 1/8" after carpet
Rack sides: 7 1/2" x 10
Top corner bracing: 3/4" x 3/4"
Bottom and shelf bracing: 3/4"w x 1/2"h - This will give you some clearance for the head.
Height of shelf - 11" from inside of bottom
TIPS: When you put in the pieces for the rack, be sure to take into consideration the thickness of the carpet. I made this mistake and the rack fits a little tighter than expected. Also, drill holes for the screws before you put them in. That way you won't split the wood. Just make sure to use a drill bit smaller than the screw shank so they still bite into the wood.