is this particle board (MDF, etc) or plywood? .. if if is particle board, I would take T-Birds suggestion and BOLT the 3/4" plywood to the bottom of the cab (through the casters and one or two to the center wouldnt hurt either, making sure to secure the inside nuts in a manner as suggested or equivalent ( I like nylon locking nuts) ... I would probably shoot some adhesive into the voids created by the previous caster pull outs ... you may be able to just re-use the existing casters if they are decent ...
... just what you wanted, more weight added to your 410
Edit to add: I like the T-nuts idea as well, that way you can re-tighten externally once the cab is back together ... the only caveat there may be if the cab is very dense MDF type board, the points on the T-nuts may not 'seat' as well as if it is plywood ... BTW, what ever you use, make sure you use bolts rather than screws, so you can get a socket/wrench on the head and not risking stripping a screw head ...
... just what you wanted, more weight added to your 410
Edit to add: I like the T-nuts idea as well, that way you can re-tighten externally once the cab is back together ... the only caveat there may be if the cab is very dense MDF type board, the points on the T-nuts may not 'seat' as well as if it is plywood ... BTW, what ever you use, make sure you use bolts rather than screws, so you can get a socket/wrench on the head and not risking stripping a screw head ...