I would pony up the money for a couple 10 round mags honestly. I used to live in a ban state and most states will say the mag has to be permanently modified to hold only ten, which often means welding riveting or epoxy to make it incapable of disassembly making cleaning and maintaining impossible. There is/was a company called mag block dot com making limiting blocks you can insert to limit capacity but you still need to seal up the mag some how to be compliant with law. Having extra magazines around is never bad and if this is for carry you should be carrying a reload any way. While there is some issue with 10 round factory mags in some guns they would still be likely more reliable than anything cobbled together. There was some complaint for example that glock 10 round mags were not reliable but mine have been fine. I at first after I moved figured I would get rid of the gen round mags, but several types of competition I shoot ( uspsa production idpa and gssf) have a gen round limit anyway so have continued to use them.