If you're going to convert a 5.45 polymer magazine, why not convert Bulgy 5.45's? The magazines are less expensive to start with than the Polish 5.45's. You'll need to Robarm follower and will need to file out the some of the inside of both lips at the top of the magazine to allow for the 5.56 cartridge shoulder. Use the converted 5.45 magazine that came with your rifle as a guide. I did this to a couple of black Bulgy 5.45's and they work perfect.
FYI, it is not a given that the Bulgy waffle 5.56 magazines will simply lock right in your rifle. I had to file a little off of the top rear of mine to allow them to clear the back of the magazine well. They also work perfect.
Ideally, I'm told that your best bet are Polish Beryl 5.56 magazines. They're suppose to just lock right in and work perfectly. Unfortunately, they're scarce so converting Bulgy 5.45's is a better option when cost is a factor. They're current production magazines, so I'll bet we see more of them at a reasonable price in the future.