I found out that C-Products makes Duramags for .204 Ruger, but only in 5- and 10-round sizes as far as I can tell. I'm thinking of picking up a 5-round for public land and two 10-rounds for target shooting. People say their big .223 mags work for .204 Ruger. I may get a couple. If they don't feed, they'll still be good for .223, and you can't have too many.
I used PMags in a .204R I built a few years ago. IIRC I could get about 7-8 rds in a 20 rd Pnag before things got too tight. Isn’t the shoulder on the .204 case a little more forward than on a .223? This caused the .204 cases to start to bind at some point due to a small rib on the interior sides and towards the front of the pmag. Seems like I read somewhere folks would disassemble the pmag and shave the rib down on each side to allow easier loading. Regardless of how many I rounds I was able to stuff in I don’t remember there being a problem with the feeding.