3D printer would be good but I didn't have one. Although not sure how good the 3d Print material can stand up to heat as well.
as for being close to the firewall I have not seen that area inside the car getting hot. Also the metal mount in the car is lifted about 2" from the carpeting of the car so the spacer is not in contact.
For Stopper I used the cutout from wood from making the spacer so it's the same thickness
Here is a thread I found on melting temperatures of different 3d Printer materials. It doesn't look like they can stand up to high heat either:
https://3dprinting.stackexchange.com...d-printed-part