Steve I understand the home shop method & it will take time and will work with the plasma cutter, but I'd use nothing heavier than 14 gauge. for the fins, much easier to fabricate , clean and look well when complete. A small wire welder with .030 flux wire will do well. Four short stitch weld's on the bottom of each fin will hold. Clean up as you proceed and cut an aluminum spacer plate to keep the fins perfectly parallel and uniform as you repeat the process till complete. Nothing wrong with using heavier material if you wish, less risk of burn through. Costs can be lowered by using drops, cutoffs that are available in sufficient quantity so do the math on each fin and piece required I used 3" tube, 2.5" would have been my choice if I took the time to source it. I used what I had. 5" fin diameter, cylinder tube length 6.5", crank case width 5" and 9" crankcase diameter but that doesn't include the shape required for the cylinder base plate ^ shape that' s up top. Your cylinder head or heads will ultimately dictate upper fin diameter and completed engine height. My dimension's yielded a motor height of 19", which I would hold to 14" maximum if using a standard size Sportsman frame with motor housed completely inside the loop. Also I'd also keep crankcase diameter at no more than 6" diameter, which would adequately fill up the loop with room for mounts and also allow a bit more room around the bottom bracket.
The use of tubing and case bolt through case construction was my initial design plan and will absolutely work perfectly with fewer headaches involved and look great.
My desire to build the F-style motor at 1 to 1 scale really complicated my build even with the custom measurements of the frame. Good news it's going to work and look as planned!
My case is 5" wide, my design allows the motor to extend a bit on each side, so motor selection enters into this, but that's not for this post.
I use a 3/4" jack shaft and it's over kill for sure, a half inch jack shaft would really save space in the case & 6" diameter case doesn't leave much room. Again another post required later.
If you bind all the fins together, after initial cleanup, the edges can be more easily and uniformly finished, indexed and numbered as to assembly position & ordered from head down. Depending on cylinder design some fins may be of significant size and finished shape. If there is to be a push rod or more inside the fins pre drill the pilot hole or holes while you have the fins clamped together in a drill press to keep them straight, of course if you've an adequate drill press you can finish in the press as well or do them individually.
Planning and setup helps, but shouldn't be a problem for you.
Rick C.