This is looking good! Certainly better than the first balsa build I did.
About the cowl: If you used PLA, don't. It sucks at basically everything except being cheap. It's brittle, and heat will warp it out of shape in no time. Glow engine exhaust also isn't particularly nice to it. I think it will soften it over time. It doesn't matter how well you deflect your exhaust away from the cowl; engines have other ways of expelling fuel and oil onto everything surrounding them. This is why you never want a cowl that seals flush with the fuselage bottom as oil will slowly accumulate in it and soak into everything. At the very least, drill a hole. My strategy would be to if you want to use a printed cowling, use something more hardy like ABS or PETG, if it comes down to the two I would say PETG because it doesn't offgas like ABS does.
Another strategy would be to print a cowl pattern with whatever material you want; then use that to make a negative mold out of Hydrocal or something and lay up glass on that, which will give you a much stronger end product. In a pinch you can also use the pattern itself as the mold (just use a healthy amount of release wax!) though you'll have to do much more sanding to get a smooth surface in the end. If you ditch the printer entirely you can just make the cowling plug out of XPS foam but beware that in that case you can't do the second option; if you use polyester resin, you must make a negative mold as polyester resin has a nasty habit of destroying most types of foams on contact. You can however get around this by using epoxy resin instead of polyester.
I would also suggest at least sealing the firewall with glass resin. Any wood left unsealed will eventually become soaked with fuel - this is one of the reasons we cover airplanes in the first place
Glass resin does this job phenomenally well as it's thin enough to be brushed onto surfaces and if you thin it further with denatured alcohol it can permeate into the wood grain itself which will make fuel totally unable to creep in. You can use cloth here too if you really want the extra strength but any airplane where the specified firewall isn't strong enough without it is poorly designed...
