A Summary So Far
Unless you have a license to possess prohibited devices, you cannot possess an assembled magazine that has a capacity greater than 5 rounds. Even un-assembled, an overzealous LEO may consider the parts sitting inside one box un-assembled to be prohibited. Ghey, yes.
Magazines are on the BATF exportation list for SKS rifles. It is considered a compliance part. You will need an export license to get them legally. GFL with that. You will have to order a few hundred to justify it.
You "can" for sure ship them to a PO box in Buttsecks, Nowhere, drive there, and pin them to make them legal to possess in Canada. You're still stuck with the export issue. Have people shipped themselves magazines, barrels, etc., into Canada this way, SURE THEY HAVE. You won't find anyone admitting it though. Can you be held accountable for doing it by the BATF (not the Canadian government they don't give a crap) PROBABLY.
Your SKS45 was designed to properly feed ammo with a non-detachable box magazine only. These magazines will reduce performance of your SKS's ability to feed properly. While I am not dissing SKS45 owners who have these, more than not people will tell you they don't work properly.
Think of the cost comparison:
SKS45 Russian + Duckbill magazine - landed approximately $360.00. That cost may include having your name added to the U.S. Fed Watch List which will at the least make your U.S. travel "difficult."
SKS-D - landed approximately $450.00 here on CGN when they come up for sale. A mere $90.00 more than an SKS45 with duckbill magazine. The rifle is "guaranteed" to feed properly and you have no concern of being put in a precarious spot with the U.S. Feds when you travel to the U.S. And, the magazines for them are abundant in Canada because they are imported from China.
If you think saving $90.00 to convert your SKS to a detachable magazine format that doesn't work 100% of the time vs. getting an SKS-D is favorable, then you should definitely go for it.