I would pull the bottom drivers, take all casters off, get nuts and bolts long enough to pass through, use the oversized washers that available at any hardware store along with threadlock glue and permanently bolt them on. Washers and threadlock on the nuts INSIDE the cab.
This, or...
(What I would do)You could pick up some Adhesive/ sealant, 1/4" T-nuts, bolts and washers at Your local home improvement center and repair as follows:
* Remove the two speakers from the bottom side of the cab
* turn cab upside down, or on it's side to perform work
* Measure the overall thickness of the wood
* Buy Bolts and T-nuts, long enough for propper clearance of wood and casters.
* Drill existing mounting holes to fit the T-nuts
* Apply, a bead of sealant to T-nuts prior to installing
* Re-install casters using the t-nuts and bolts, tighten as needed, and/or use lock tight conservatively if it suits You
* Re-install speakers, and other hardware
* Enjoy