Well Done!!
Great use of an unusual space! I may adopt your PVC idea but affix it to the ROPS (for longer tools). I guess that water could penetrate if it were left outside and it rained. Perhaps you could, "incorporate an O-ring into your ingenious invention". One that would slip over the 4" PVC and would be retained by the cap, A Dremel tool to divot the circumference of the cap to get that snug fit.
I think that you could market this as there are so many loader Manufacturers that utilize this same (empty hole) on their FEL. I would buy one!!
May want to patent it and sell your idea, it is a good one.
Is yours deep enough to hold a chainsaw file? That was the first thing that popped in my head for a use.... that and spare chainsaw chains/rag's.