Ok - so it looks like its about turn time & back to the drawing board.
Are we talking 50mm x 3mm mild steel box?

It'll be bolted, not welded as I don't have the gear for that - but I do have a pillar drill & some taps, so should be able to cobble something pretty sturdy together.

Screws - i was looking at a Marchant Dice package on ebay...but given that i'm now looking at supported rails from Chai, i think it makes sense to get some screws from him as well - i'll need to look at what he offers & the pricing. If the steel does work out that much cheaper, then the budget shouldn't be an issue.

I'll call my local metal supplier tomorrow to get a quote for the steel box & then start drawing something up.