Decided to try making my own thick bar dumbbells. They are now 50mm thick, the same diameter of the outside of an olympic bar.
Bought some 50mm PVC pipe, and pipe insulation.
Just wrapped the middle of the dumbbell with the insulation, and slid the PVC pipe over the insulation. It is a pretty snug fit, so hopefully everything will hold up.
The only disadvantage I see right now is that the pipes are not 100% centered on the dumbbell.