Build a new head for a BP series 1...

[m_c]

I would of thought a ribbed design would be better for the main section. That way you could get a reasonably nice L/Boomerang type shape going from the main body up to the vertical surface for the rails.

I could be wrong, but I have something in my mind telling me U-shapes are just about as good as an enclosed box section, but it's not really something I know that much about.

I think given the size and potential cost of the project, I'd be spending some time drawing things up in 3D software and running some FEA to see what works/doesn't work. A counter balance might be a reasonable idea, but a couple gas struts would probably be enough and reduce complexity. A quick google shows struts are available with up to 2500N (~250KG) force for not that much money (14mm rod with 500mm of stroke is available for £40+VAT from - https://gasstruts.metrol.com/fixed-f...ut-f-sd04.html )