Thanks, Simon! I was very careful, but I did deviate from your advice a bit by using a drill guide. I attached a magnetic level to the guide, propped it up flush with some thin cardboard, and tried to keep it as stable/even as possible. I figured that was a touch more reliable than eyeballing it given the curve of the body. Anyway - I believe it was a success! The posts seem to hold up the stop piece at the correct angle although I did not tap them in all of the way yet.

That brings me to another question - neither the bridge posts nor the stop piece posts slide into the holes. I assume that is because they need to be gently tapped in. The manual says to wrap them with tape and slide them in, but I am unable to get them pushed in with tape and it seems counterintuitive to cram them in the holes with the tape if they need to be tapped in with a hammer later. Do I need to use force? I'm still in the "mock" build phase as I haven't yet found the time to get really into this.