For something like that with a wide thin base, in PETG, I find that printing it with a 3mm brim helps reduce the risk of corners lifting part way through.
Some guidance here from the HMRC:
https://www.gov.uk/hmrc-internal-manuals/employment-income-manual/eim23120
The test now is whether the construction of the vehicle is primarily suited for the conveyance of goods or the conveyance of passengers.
If they've not done it yet I'd suggest the OP contacts Reynolds directly - they also do products for motorsports and motorcycles, so it may be that they already have something in their catalogue which you can repurpose.
https://www.reynoldstechnology.biz/contact-us/united-kingdom/
Just follow the link... https://www.lampspares.co.uk/15mm-copper-pipe-stop-end-with-10mm-allthread-plu87172/ - there are four more pictures there.
It's basically a plumbing end cap with a 10mm thread insert already fitted, onto which you can thread a lamp holder.
Prusa have a basic pricing calculator on their website here:
https://blog.prusa3d.com/3d-printing-price-calculator_38905/
If you're planning on doing this for anything other than hobby LoLs, where at best you can hope to cover your material costs, then you really need to think about initial...
Dumb question perhaps, but have you tried their UK distributor (https://www.euroroutensl.co.uk/), or reseller (https://www.zen.co.uk/smallerbusiness/superfast-fibre-and-broadband-packages/routers/), to see if they can process the return for you instead?
You can usually get different nozzles / needles sets for the double-action airbrushes which will give different widths of spray pattern.
For those who don't know, with a single action airbrush you can only vary the air pressure by how much you press the button, with the amount of paint drawn...
Quoting from somebody more knowledgable than me on another forum:
body tinware 1.0 - 1.2 mm not much difference to the result, but the 1.2 mm is better to weld and work with and gives good support when butt welding /letting in;
1.5 mm for steps, lip and edge repairs, inner arch repairs...
Just put the holder straight on the scanner and trace around the image - that way you can eliminate most of the error associated with hand tracing, particularly in the corners. If you put a steel rule face down on the scanner next to the holder you also get a visual check scale.
We have one of the SumUp Air readers - it does need to be used in conjunction with a smartphone, but there's no monthly cost, it's straightforward to use, and it does the job just fine.
Re. cutting off the return / upstand, if this is just at the base of the opening then my first instinct would be to use a router. I'd probably build a long timber 'table' with some 3x2" or similar that sits over the frame on either side of it, which the router base can bridge across to allow me...