With a router:
1. Set up edge guide to remove the inner 1/4" lip of the cab
2. Set bit depth to just a hair above the particleboard bottom.
3. Ear, eye and lung protection required now.
4. Rout out one long side and one short side from cab.
5. From the inside of the cab, separate the cleats from the bottom of the cab with a chisel. They are soft pine, so a few may just explode. Be careful not to chew up the blade of your chisel by hammering into staples.
6. Kick out bottom of the cab.
7. Remove metal Ghetto Plate (if present) from the bottom and use the bottom as a template to cut the replacement. Use 1/4" Baltic Birch plywood, not more junky particleboard or Z-brick.
8. Once you have the bottom panel cut out, clamp the old bottom on top. Using a Pattern Router Bit, cut the circular Speaker and Switch holes out of the new bottom.
9. Take some 2x4 pine scraps and run them through the tablesaw at 45* to make replacement cleats. Make extras, because you will certainly need them in the future.
10. Flip cab again to upsidedown and do a sanity check that you cut the replacement panel properly.
11. Run Tightbond3 glue around the edge of the cab and set bottom panel in place.
12. Using small brad nails/staples, attach "window screen molding" to secure the bottom panel where you routed out the 2 sections.
13. From the inside of the cab, wipe and glue drips with a wet rag.
14. Brad nail/staple the internal cleats (I know the glue is enough, but it's authentic).
15. Paint Ghetto Plate battleship grey and reattach.
16. reinstall switch block, speaker, power supply.....
window screen molding
pattern router bits