A modified mag is the same a producing a mag, and needs to meet all legal requirements. If the mag is un-modified, and made specifically for a firearm that has a greater than 5 round legal capacity, than you are good to go. If it is deemed as a "dual use magazine" by the RCMP, even if it is indeed designed for a bolt action, or pistol, than you are back at square one, despite having no law on the books to support their argument. Now, if you have all the time in the world, and a huge legal fund, law team, and interest, you could make a good fight, but the judge will probably side with the RCMP firearms "experts", with no process for appeal. The same experts that make a 22lr that looks like an ak-47 a variant, and a 12ga that looks like an Ar-15 a variant, despite sharing no parts.
Good luck with that.
Modify the gun, not the mag, and keep your lawyer on speed dial.