If you can take a stick and use it to limit the shells in a shotgun, a block of wood is more than enough...as long as it does the job.

besides, do you expect a game warden to know which mag is factory 5 round and which one has been set up by you to only accept 5 rounds?

there are plenty of mags out there, take the M1A 5 round mag but it is in a 10 rd body.

just do it yourself and don't worry about it. every game warden I have run into will only check operation and will load the mag, as long as it doesn't take any more than 5, you are GTG.