I've used openscad to modify the bottom tripod brace part to make it fit my 40mm extrusions. The new part has a slightly different "clip" to match the different profile.
It was a terrible job to do in openscad but it fits nicely:
Since I printed another bottom tripod part I now had enough tripod parts (with my earlier top and bottom test prints) to line them up on a 200x200mm heat bed to get an idea of the scale of the finished printer footprint:
My optimistic hopes that the dual 623 bearing carriage would fit seem to be unfounded:
I'll probably try using some washers (or half nuts) to modify the position of the bearings to see if that is enough to stop the PLA touching the extrusion but failing that I'll have to attempt remodelling it.