Anthony,
Good question - we haven't yet! However, had thought that we could get some plastic/rubber blanking grommets(I think we used a 29mm cutter) or a small teak-faced-ply (or plastic) plate covering both holes and held in place by self tappers. The only issue is that we have holes only on the port side, so not aesthetically pleasing.....!! The fact that we haven't yet got round to covering the holes probably means that they are rather unobtrusive.
Incidentally, the nuts holding the organiser were ny-locks(?) which may be why they become detatched easily i.e. extra pressure needed to undo or start tightening.
Regards
Chris Cobb
235/48 "Tarakihi"