I've seen 3D prints being processed with blue light, so they are extremely sensitive and the tube will eat the sockets within days

They may become just "overhardened",

also they won't be happy with the heat because 3D filament is designed to be extruded with rather low temperature around 70°C, just like normal hot melt glue.

Not true, when generalizing. The temperature depends on the exact material used and it varies a lot.
Yes, some materials suffice with very low heat (the PLA is convenient there), but especially the ABS requires way above 200degC, so beside the high temperature nozzle a heated bed/chamber extruder to work (otherwise it peels off way before the work finishes).
Many  people are equipped with these high temp extruders, so may work with the ABS and then it can stand 120..150degC (150 just for a short time) as common for the ABS plastic.

And some materials are actually hardened after the extrusion is done (resins, activated by the nozzle heat,...), so after hardening are able to withstand way higher temperatures... 

But still, based on how the object is formed by the layered filament extrusion, it always yields porous structure.


Of course, using the extruder to just make a mold and then make the final product into that is material wise the best way to go (generic low temperature PLA and basic extruder without any bed heating or so is sufficient), it is just a bit more complex to design he mold parts with all the limitations (it is possible to form on the extruder, at the same time it s possible to use as a mold and extract the final product from it,...). Sometimes it is easier to just carve the mold form e.g. wood the "old school" way... All depends on what tools and skills you posses...

If I think about it, you actually don't need to design and print a mold, you just need to print the object and make a cast of it with proper material that can handle the incoming material, for example flexible silicon, clay or sand, then you just pour the material and make copies of it. Also the carving suggestion is good and it will work as long as you have enough wood knowledge and the model doesn't have much detail. And if you are nuts enough, you can make steel molds and a hot plastic injection system and you will make your very own socket factory. 

I think if you are able to use ABS, it should work for low heat lamps.
But the UV could be a problem. I don't know if the filament materials have any UV stabilizers in them...

And the porosity of the extruded print would be an insulation problem (it will hold moisture and so form current paths).
So the print may need some good sealing treatment.

One thing I do know about ABS is that the stuff that is dyed black always uses carbon soot, which blocks light including UV from entering and degrading the plastic. I would definitely use black if I was making a lampholder.

Also the carving suggestion is good and it will work as long as you have enough wood knowledge and the model doesn't have much detail.

You would be surprised to how much detail the wood may go. It is often the material of choice, mainly when carving molds by CNC mills (as even the cheap bits tend to last very long before needing sharpening).

Or for hand carving and low temp curing epoxy, a wax could be easier material to work with. It is completely isotropic material, so wont misguide you when cutting by hand. The drawback is, you must be sure the resin hardening wont create too much heat to melt or deform the mold before the resin solidifies enough to hold its shape on its own. If for single use, it is then easier to remove: Just melt it away..

I think the candke wax is the best option. You just need the 3D printed object treated with something for the wax not to stick to it and then you let the object sit into the molten wax and while it solidifies, and the resulting wax cast won't stick to practically anything, you can then use a huge variety of low temperature curing materials, them for take the object out you destroy the mold, melt it again and continue the process.

You could always use this method. 
Making the mould from an existing socket.

I have been lost in the last comment on the previous page... So reposting

FR4 circuit board for the socket face (flat face with 2 holes for the tube pins) that comes in contact with the tube cap, and any non-demanding 3D printable plastic for the back shell enclosing the socket

Overall socket design like in "pull out" British lanterns for T12 like GEC Europa (Gaash 2000), Fitzgerald Lightpack (early 90s 1 tube version), and similar

If it's anything like the T12 HO lamps, then around hot to touch. Unfortunately I don't have any lamps or a ballast to test.
The F40T17 lamps would probably run significantly cooler due to the lower power loading.

I have a F90T17, but I only tested with a F48/96 T12 HO ballast
The F40T17's don't get all that hot - think a F40T12, but running a bit cooler.

I'd love to find some T17 lampholders (or could probably have someone  print them for me if I had the plans). The 'metal parts' would be a bit tougher..