Carpenters, amateur carpenters, cabinet makers, and quality craftsmen,
LOOK AWAY NOW!!!
I got a piece of 3/4"
OAK ply wood, cut it so that it fits nice against the tolex. Then I used a liberal amount of liquid nails and clamped that sucker on there. Then I used 3 Ikea hex wood lag bolts (I love the HUGE threads on these) and a bunch of wood screws to make sure that it is
firmly attached to the base of the cabinet. Then I installed the casters in that.
I figure that the damage to the original plywood is irrelevant in this solution. The liquid nails and plenty of screws are holding these two 3/4" boards together pretty damn tight. In order to repeat the damage, they'll have to either destroy both 3/4" oak and 3/4" birch, and break the bonding strength of the liquid nails and many screws. This has got to be sufficient.
I painted it black (instead of red) because it'll be dark on stage anyway, and I don't have to match shades of red this way. I also took the time to repaint the grille, which had already become nicely "reliced". The cabinet is still really tall and somewhat unstable on these big 3" casters, but still very light.
I'm happy with the results. The gigantic thundering red rock machine is back in action!
Oh, and not 100% finished yet - I will be painting over the exposed wood with black paint, so you won't be able to see any damage unless you really inspect it closely.