I'm a long time hammock user and had made my own tarp before there were many designs to copy. It has a diamond shape and it was the right size for a slightly smaller hammock than the one I now use, so it could be slightly larger but as far as needing any end doors, etc. I've never found it to be lacking in even the heaviest downpours. Pitch the tarp close enough to the hammock and any rain coming from the ends just blows underneath. The trees themselves keep most of it from reaching you.
The evolution of my hammock use also started before there were commercially available underquilts. When I started it was with a net hammock and for both insulation and comfort I just used a Thermarest 3/4 length pad. The net hammock caught the corners of the pad and locked into place. The hammock I replaced the net with was all nylon fabric and at first I would wake up with the pad beside me, rather than under me. One logical solution would be to get an underquilt but instead I started putting the pad inside the sleeping bag. It's not a mummy shaped bag so it worked pretty well. I never have trouble staying on top of the pad. The advantage to using the pad rather than the underquilt is that the pad works too when I have to sleep on the ground. It happens sometimes due to a variety of reasons, camping at or above treeline could only be one of them.
This is in the municipal park in Cap Chat, PQ and I had stopped to use a picnic pavillion and afterward it was just too dark to go in search of a better spot. I guess to each his own, but I like the flexibility of a one-item-serving-multiple-purposes.