I know how discouraging rounded fuselages can be, I designed an F4F early this year and it took a lot of head-scratching. I used posterboard over a series of formers. I actually started the design process for the skin by taking copy paper and using trial and error to get a rough idea of the shapes and dimensions before transferring it to posterboard for another round of trial and error. I wrote myself little notes on changes I needed to make along the way to help remember everything. After I was happy with the rough shape, I took all the pieces I had made and scanned them with a copier, and then traced them into Inkscape (which is a fantastic program for simple plane design). The final design wasn't perfectly perfect, but it was pretty good. One issue I had was the posterboard bowed outward a little, so I used 5 or 6 rings of posterboard to minimize their length so the bowing wasn't as noticeable.
The rear posterboard hasn't been put on yet so the rearmost formers are visible. The front formers are attached to a box fuselage on the inside, and the rear is designed like the FT P-47. On top of the wing, the bottom edge of the posterboard pieces bowed outwards a lot because they aren't a full tube, so I put a few pieces on each side that push into the wing and act as an anchor.
Basically, as soon as I simplified to the process by doing trial and error in real life instead of on a computer, it all worked surprisingly well.
Good luck if you continue this project!
Sam