This HOn3 4-6-0 frame is designed to use 13mm drivers (45" in HO scale) with 1/8" axles. Suitable self-quartering "Romford" drivers are available from several suppliers in the UK - be sure to order the HOn3 axles. The basic dimensions are the same as an ET&WNC 4-6-0. The front coupler box will allow for a kadee 714 coupler in its own box. There are holes in the frame to use my HOn3 motor cradle/firebox (separate part). Most of the screw holes are for 2-56 screws. You can tap the holes on the pilot truck for 0-80 screws to secure a cover.
You can print this in the basic versatile plastic, the premium versatile plastic, or the PA12 plastic. All three are nylon and very strong and durable. The basic versatile plastic will need filing and sanding to achieve a smooth surface. Note that none of these materials can be glued to easily - any details must be added with a mechanical connection such as a screw or pin. You will need to make a cover plate for the drivers - PCB works well and you can easily add power pickups using modified kadee #5 centering springs. The pilot truck frame needs wheelsets with 2.0mm axles. Cover plates for the pilot truck wheelsets can be made from 1/4" brass and secured with 0-80 screws
I've only tried this one in versatile plastic and it did require some filing and sanding of the rough surface. Premium versatile or professional may produce better results, but I have not tried those materials. The model shown is a slight variant I made to use MDC drivers I had in my scrap box, but is otherwise the same. I am using a slightly shortened MDC 2-8-0 boiler, but I am designing a more appropriate boiler to go with this frame.
It does work (see video). I also 3d printed a gearbox (not yet available for sale) to go with it and used gears from a 0.4 modulus assortment found on Amazon. The motor I used is also from Amazon - Mitsumi 12mmx15.5mm (They used to be real cheap on there, but it seems like the price is much higher now).