I wouldn't use that method to build it.? I would drop the extra top rail and just extend the uprights to get your extra height.
It will save on material and resonance because those will be 2 large drums you'll be creating. To be honest I wouldn't use 100mm Box I'd go with 80mm and thick walled.
I'd use the design Eddy showed with top rails sat on plates which can be adjusted into plane much easier. The work involved with Epoxy is quite involved and to be honest a complete waste of effort if your going to use those Cheap round rails has there will be more error in there base than the surface your fastening to.! . . . . I also doubt you'll get it any better with epoxy than using the above method and it will take much longer and not be has flexible, it will also cost lot more money.

Show us the same picture but with out the bed base so can see the bracing etc.