When you tie your fiador knot - do you use a mandrel or do you do it freehand ? Could a small mandrel be used and inserted in the bottle and worked to form the knot ? I am not a knot person - I am just throwing this idea out there...
I know that this is an older thread, but I do have an answer to your question. I started out using PLA, but soon switched to PETG. Alot of my 3D prints are used in an outdoor environment and PLA just did not stand up to the task, so I switched to PETG and have not look back. I run a nozzle temp of 230 deg C and a bed temp of 90 deg C. I also use a PEI magnetic build plate to aid in removing my 3D prints from the bed - PETG really sticks to your build plate the PEI really helps in removing your prints...
There is a niche market for acrylic templates for such items and I do not know why 3D printed templates wouldn't work. You would need to use ABS filament for your templates to be durable and withstand a lot of use. I do not think that PLA would work very well - PETG would be better than PLA, but not near as good as ABS. I have downloaded several leather template .stl files from Thingiverse and Printables and print them with PETG - they do work for my limited use
The Ender 3 Max is the filament printer that I would recommend because of the bed size needed to print a mold gun. As mentioned above PLA filament will not work well - it would probably a one time use thing. PETG or ABS filament is what you would need to use with ABS being the best. You will need a vent system for ABS due to the fumes produced when printing. As mentioned above - the CAD files needed to print a accurate mold gun are like "unicorn horns".. You will need to be able to design your own and good CAD software is expensive.
If you heat it up it will thin out. Place a usuable amount in a small bottle with a screw on lid and emerse the bottle in hot water. Let it warm up and you will be good to go...