Only 20 round I know of is the factory polymer 20.
Way back when someone in the Czech Republic was cutting down surplus mags to 10s and would sell a handful every so often on eBay. I grabbed a few for bench mags and the hack job looked good.
But never seen anyone cut down VZ 30s to 20s. A guy on the AK boards was cutting down AK surplus 30s to 20s and did a fine job, but wasn’t interested in trying VZ mags.
Personally 20s in any platform are the ‘sweet spot’ for me, short enough for bench but enough to get the job done.