The Viggen nose you see in my avatar is exactly what you propose. The FT foam construction is great and actually looks surprisingly scale except trying to force foam board to form a pointed nose is a struggle. So I designed a 3D printed nose cone which is sacrifical just like the standard foam board nose. While I was at it, I included the intakes to clean up that area as well. I have made quite a few noses and countless motor mounts to replace the ubiquitous Flite Test power pod, something I never use.
For a simple firewall, I usually make the firewall itself 2mm thick with three top and bottom layers and a 40% or 50% infill. I print thin webs aft of the firewall which fit inside the typical square foam fuselage. Usually they're about .8mm thick and extend 5 or 10 mm aft of the firewall. I usually make a series of 2mm holes in these webs so that the hot glue will have something to grip. More often than not I print a very short (2mm) web extending back from the firewall which fits on the outside of the fuselage. You can see that feature in the rendering below. This particular firewall, an early design, is more complicated than necessary, but it gives you an idea of what I'm talking about. you can see how the inside and outside webs capture the front edge of a square foam fuselage and capture the vulnerable, unprotected raw foam edges. These days I would eliminate the X web in the center. When you crash, these firewalls never suffer damage. It's always the foam which fails, so you can go lighter than you might think when 3D printing firewalls.
View attachment 141400
Typically I make four bosses to accept 3mm motor mount bolts. I make undersized holes and run a 3mm tap through them and thread the screws directly into the plastic. I find that works better than printing the threads. Or, sometimes I print a feature to capture a 3mm nut on the aft end of the mounting screw boss.
Get familiar with the loft command and the shell command in Fusion 360. They will come in handy when making nose cones for EDF or prop-in-slot jet models. On simple nose cones, you can loft a solid and print it as a solid using Vase Mode to produce a very thin and light nose cone. Or you can shell the solid body and print using normal methods.
I recently made a Bloody Baron nose. It connects to the standard square fuselage just forward of the wing leading edge. I made a loft from that square cross section to a round firewall. The part looks like this:
View attachment 141401
View attachment 141402
Then I designed a Fokker DVIII style cowling which is printed separately and glued on after the motor is screwed in place. Those rails extending aft support a foam "shelf" which holds the battery and ESC.
For best results, give some thought about the print orientation EARLY in the design process. I have a tendency to make my parts very strong. Consiquently, they are over built and heavier than necessary. I have overcome that bad tendency for the most part, but I'm always fighting the urge to add another web or another fillet. Try to build hybrid parts lighter than you think is appropriate. A horrible crash always results in a pile of crumpled foam and a perfectly intact plastic part.
I use a .4mm nozzle with .2mm layer height. When I draw a thin part and think that a 1mm thickness would be OK, I now specify a horizontal distance of .8mm or sometimes .4mm. In the vertical, rather than go with 1mm, I specify .6mm when I can. Keep fighting against making parts too strong (heavy).
Of course things like control horns, tail skids, wing tips, servo arm extensions, arrow shaft joiners, battery trays, and stuff like that are easy enough if you give them a bit of thought. Again, lighter is better.
I print all my parts in a "tough" PLA. I find eSUN Pla Plus (also called Pla Pro) to be a very good product. It's much less brittle than ordinary PLA. I print it at 235 degrees for ordinary parts and 245 degrees for single wall parts. It gives great layer-to-layer adhesion and a good surface finish. It's available in many colors at a reasonable price from Amazon.
