Thinking about an 80% just for the heck of it.  But the KTordnance ones don't have the mag release slot cut already.  I guess it's because they partially drill some of the holes already.  Couldn't find anything on their website and the owners seem to still be in the process of moving.  Didn't find anything on the search here or on google.

I can get a jig from them and use a drill press at my buddy's autobody shop, so that's not a problem.  But how the heck do I cut the mag release slot?  

you need a mill.

Dremel tool.  They're great for everything.

I would say you need a mill. Dremels are good for a lot of stuff, but I dont hink I would trust it on a lower.