Start with a carton that most nearly fits your amp with a few inches to spare on each side. Rummaging through the dumpsters behind GC is a good place to look!
Go to a craft store and buy four big blocks of styrofoam. Take it home and measure how big a cutout you need to create a tight corner pad for each corner of the amp. Get a sharp knife, and get busy!
If you're REALLY stingy then look for styrofoam corners while you're dumpster diving at GC.