Drilling your own trunnion holes (especially if your trying to do an underfolder without a template) can be a real pain. But I'll tell you what is making this a breeze now. I bought a trunnion drilling jig a couple months back. (It's not the one shown below. But the same idea.) I threw it in my build tool box. And forgot I had it. Had never used it before. It's basically like the one below. Except it expands more with what looks like a big concrete bolt hole holder thing. You know those things you put in a hole in concrete or cinder block to hold a screw or big bolt? Like that. Except BIG. You screw the bolt in and out to make it expand or retract to the size hole you need.
The jig is meant for the front trunnion barrel hole. But I fiddled around with it and got it to hold the rear uf trunnion. Which is kind of odd shaped to get ahold of and still be able to slide the receiver on all three sides.
The jig holds the trunnion in place so you can center your bit perfectly in the drill press. Then slide the receiver on and drill the hole. PERFECT placement. No measuring. No tape templates. It's a pretty common jig but mostly used for front trunnions and AKM full stock rear trunnions (has screws to hold those in place). I don't know if the one below would have big enough couplings to work on the UF rear trunnion. But even if this thing saves one $100 Nodak receiver from getting mangled with bad placed holes it's worth it's money. It's definitely worth it for the time it saves you. And the underfolder has some pretty non-typical holes to be drilled. And this things makes it so you can place them perfectly without any measuring or guesswork.