Most low-end 3D filament printers can print in either PLA,  ABS,  or even nylon.  PLA prints are known to distort in a hot car.  ABS is extruded at around 250C.  I think nylon is a bit higher.  I would think that ABS would work well.    There are now some low-ish cost  (like under $3000) stereo-lithography printers available that print by scanning a liquid resin with a laser.  Some of those resins are good for higher temperatures.  The low end resins have a glass transition temperature of around 100C.

Another possibility is to make a mold and cast the parts out of epoxy.  Epoxies that can handle over 400C are available.

Hi,

for me, the obvious solution was a heat-resistant spacer - a EPDM O-Ring
in my case - to fix a hat which is a bit wider than required, which was
a simple dustproofing cap for a vacuum port (a bottle cap might also
work). The thin air layer in between is not expected to do too much harm.
This seems to be PP in my case, but as it is already a bit insulated,
not much harm would be done even with a hotter Ref. On the underside
(low distance to wall of metal box), I had either a piece of foam or
nothing - i forgot. Frank, do you still know as you inspected it?

Also, would everybody who wants a heat resistant cap please stand up -
thank you - and now would everybody sit down who expects less than 50
deg C inside the reference box ? Go for simple isolation, invest in the
right resistor to do the 45/55 degree hack and enjoy your lower drift.

Epoxy formed in a silicon mould would be the way I go for heat resistant
insulation parts.

Hendrik