I have a 10 and a 20 shot fixed mag for my Norinco SKS. Takes about 15 seconds for me to change them out.
I suppose that if you live in an area legally limiting mag capacity to 5 shots, it would be very easy for a supplier to block off a 10 rounder to 5.
BTW, Norinco offered fixed 20 and 30 round magazines at one time. I'l betcha anyone who wanted one could find it on gunbroker or at a large gunshow.
You can also get an AK mag adapter for the SKS, but everyone I know that tried them had lotsa trouble with them.
As much as I love hi-cap mags, I've always been of the philosophy that if you don't have time to reload from a stripper clip after firing 10 rounds, you've got bigger troubles than mag capacity!