I did these fillets without the aid of masking tape, and found that it wasn't that difficult, and saved a lot of time futzing with tape. So, I will do the next set "sans tape" as well.
Here is the template screwed to B3 ready to cut the second hole. I used a panel router (gotta love that little guy!) with a 1/2" top bearing bit that is 3/4" long.
I am planning on hinging these two hatches - I will post a picture of the hinges next time.
I plan to glue the mast trunk to B3 prior to assembly into the hull - it is much easier to do this stuff on the bench.
I have finally settled on a pre-coating regimen that seems to work well. I put the first coat on one day, let cure overnight, sand the next morning (first coat always seems to be rough, so wants to be sanded), and then put on the 2nd and 3rd coats that day, the 3rd coat going on after 2nd is still tacky - after about 4 hours with the medium hardener I am using.