I’ve always kept it simple- tape it, squeeze it down well then just slice round the hole with a good sharp scalpel blade. No mucking about with crosses or anything.
That, and using good valves. Which ime means, something that looks like this:
https://images-na.ssl-images-amazon.com/images/I/613p6WWEjJL._SY879_.jpg
I did have one rim they didn’t work with (a BOR of some sort) but for everything else, these are the best imo. Why some valves are square I don’t know, it’s a round hole.
” I’ve even tried small rubber o-rings between the rim and valve washer”
Doesn’t help, this- the seal has to be on the inside of the rim, all you’re doing is trying to seal the outside which has another 32 or so unsealed holes in.