You can put it on just about anywhere so long as it's legibly sized and in a location that can be found without disassembly of the firearm.
I've seen it put under the original SN, just behind the ejection port on the side of the receiver, and I've always liked where ARC marks receivers on the ejection port lip. You could put it there as well.
Someone with a fiber laser can make short work of it for you, if you don't have a local place you can give us a call. We charge $50 to burn an NFA item, plus return shipping if we have to send it somewhere.