yeah the gantry setup with just extrusion is an issue.
That's what I like about the sapphire.
Base is a solid box of 2.25 mm steel and the top gantry plate is a single slab of 6mm aluminium with the rails bolted directly to it.
It's the most rigid thing I've ever seen.

You should be able to make a top plate - must have the cutting gear at work.
And didn't you recenty invest in a plasma cutter ?

And the belts actually don't need to be 'tight'. Because both motors are always pulling together often in opposite dierctions. There always seems to be tight areas and slack areas.

I will tighten mine up slightly with the new connectors - but given I can knock out a smooth working 40mm iris box at 150mm/s with the direct drive setup - they are obviously not as loose as I think they are :-)

lol you deliberately picked a printer you NEED to rebuild didn't you :-)