P is for Print. You looked very comfortable doing the design on the laptop. I know it's your core skill. Less Play and more Production needed. Show us a list of the other things you are going to print for this car. More experienced printers are posting good tips that should reduce your learning curve by quite a bit. Take notes.
FYI if you use a paint stripper that contains methylene chloride (dichloromethane) you can solvent weld PETG or PLA plastic together. It's effectively the same as welding it with the printer pen but probably a bit less time consuming.
I was going to suggest venting the room with the 3D printer due to the recent talk about VOCs involved with 3D printing but then I saw you sanding the part with no mask in said room. Safety is not the number one priority it seems.
A better way to joing piece together is to get a cup of ACETONE and stick in a bunch of ABS filament or ruined prints, it will melt into a goop, you can now use this to glue pieces together, bonds super strong.
If that is a mold, a form , or an armature for external use…why not make it porous or matrix status so that CARBON FIBER SKINS CAN OVERLAY AND REINFORCE. ..OR…IS THIS THE CHOICE BECAUSE CARBON FIBER STUFF IS TOO DIFFICULT..OR…?