I have a double skinned 26" trials rim that I use as part of a halt fat build.
I use a suitable rim tape.
Because of the nature of the cut away sections and the double skinned "ness" of the rim a void is created for dirt to build up in.
I've not noticed it be a problem but then I don't use it a huge amount.
I've seen a couple of fat rims where the rim tape appears to bubble through the cut away sections. How is this achieved?
I'm looking at some 29" trials rims that could suffer the same as my 26" rim and am looking for a solution to a problem that may not be a problem 😆
The rim tape will bulge through the hole a bit, more obviously on single wall rims than double. I've never seen anything which would bulge enough to cover that gap.
The "bulge" is merely air pressure and is only really noticeable on a single-walled rim.
There's a fair number of folk who've drilled double-walled rims as you have, ride them on "challenging" terrain (beach/sea-water) and seem to suffer no ill-effects.
I think some don't bother with tape - as long as there's no sharp edges it's OK at 10psi and just bulges a bit (Mrs)
Cheers all 8)