Just got back from a long business trip and saw your post this morning. I don't know how far you have gotten during the past few days, but here is my nickel-

I recently completed the cowling on my Nobler, which looks similar to the one you are working on, and I also fitted an inverted 46FX. This was my first cowling and I am quite pleased with it.

I had the same problem with the tight fit but kept at it and eventually managed to get the cowling to slide around the firewall leaving an adequate gap in front for the spinner. And I do think that leaving a cowling material all the way around the firewall adds strength, but I made the hole quite big to make sure I had adequate cooling. I drilled two small holes just large enough to get an allen wrench through to enable removal of the muffler. I don't think they detract from the way it looks. I also drilled a small hole to enable adjustment of the low speed screw on the carb.

I used a 3-tube tank setup with a filler valve functioning as a fuel dot. The inverted engine does tend to flood if I am not careful, but I found that if I stop filling the tank as soon as the level reaches the overflow (tubing to the muffler), that there are no problems- no clamp is needed. I first try to start the motor by hand before putting the electric starter to it, just to make sure. Once started it runs great- it actually runs smoother than my other two 46FX's for some reason.

I have at least two dozen flights on the plane and haven't had a need to remove the cowling since the initial setup. There are some pics in my gallery if you want to take a look.