I emailed the DOW about this wnen my son wanted to hunt with an Enfield with a 10 round mag, here is their response:

Robb, thank you for your e mail. This is the answer from CPW Law Enforcement.

A magazine that has some type of internal block that would require dismantling the magazine to remove it would suffice as well. This is no different than a plug in a shotgun which is not a permanent blockage but does require dismantling in the field to remove the plug.
Therefore, a block in a magazine that is between the spring and the follower or between the base plate of the magazine and the spring would be O.K. since it would require removal of the base plate to remove this "blockage". It cannot be, for example, a 10 round mag. with 5 dummy rounds in it followed by the 5 live rounds.

All that said, I think it would be easier to purchase a 5 round magazine.