Another thread on here overlaps with yours:
There was a link to ‘Lobster’ holders which looked interesting but no feedback as yet:
It looks well designed to me but of course I’ve never used one. There are quite a few 3D printed options on Etsy and of course even Negative Supply and Valoi are using 3D printing for some of their components. You are somewhat dependant on those making them staying the course and keeping up with demand but that’s also true of some of the major players in what is a fairly niche industry.
Those shooting film now (as opposed to scanning an existing archive) want to be able to quickly scan every frame on a freshly developed uncut film so those holders are designed for a fast throughput.
I’m also fortunate to have Durst holders from their M605 & M700 enlargers and these are good, very well made. My favourite is their anti-newton glass below and metal mask above, shiny side against the glass. The Durst metal masks came in pairs with the upper slightly oversize, on its own that shows some rebate, excellent. However they are a little scarce and even if you buy an entire enlarger they don’t usually come with a full set of different formats, I had mine from my darkroom days.
Good introduction to film holders from Vlad Srerebryany (Vlad’s Test Target) here: