I'd recommend to watch this -
https://www.youtube.com/watch?v=cxcLri3yn8Q he gives a very good guide about covering cabs. Alternatively you could just paint it black with a few rattle cans, use matt black if you do, easier to apply.
Regards fixing it together the simplest way is to use mdf, 3/4" is best if possible, cut to size remembering to include the thickness of the mdf in any measurements, then get some length's 1 1/2" x 3/4" ( I think) of roofing batten - dirt cheap, but make sure you get straight bits. Get the sides, back and front, screw and glue on two length's of batten whatever thickness of mdf your using down from the top (+ a smidgen so you can plane or sand any rough bits off the edges) & bottom etc so the top & bottom will pop in.
Use silicon to seal the joints inside and you'll probably need a box of filler. If you get plaster type filler, but mix it with PVA glue instead of water, it makes a good filler that is easy to sand off unlike some fillers in a tube. This is cheaper too.
If you want to be fancy use a router to round the edges off, you can borrow my bits if things are tight. Tips... if you have no fancy table saw, cut pieces slightly over then plane to the line, a plane works fine on mdf. Countersink screw holes + fill, paint a few coats of watered down emulsion wot you got in the shed or use watered down PVA, then paint. Doesn't look as good as a tolex covered one but it's quick and cheap.
Andy.
I'd advise setting the front baffle in a bit, say 1/2" for the grill if any. Personally I'd just get a grill to go over it. this place has lots of nifty parts for cab building -
https://www.penn-elcom.com/ like T nuts to fix the speaker etc. Actually I think I still have a jar full of T nuts and allen bolts, I can let you have a few cheap, saves you buying 50 of each.