I've a couple of sets of these carts (not parting with any though), but the plastic cap is literally a standard car valve dust cap (albeit metal), glued into a plastic disc, so it might be easy enough to make one. Failing that, get some trick/ano car valve caps and fit them over the valves, might look nice (I bought a set of pace forks that someone had done this to, and it looks pretty good). Would crank bolt caps fit? Might go and try now and let you know.