A lot of Pre Mat hardware comes from McMasterCarr, and properly so. But calling there with 'odd-ball' parts requests usually gets a polite but by-the-book response; 'see part #X, and if that doesn't work, thank you for calling'.
Mocking up something new and a web search came up empty for the part in the proper dimensions, so that begins a round of calls normally dead-ending with a polite, but not at all knowledgeable 'call center' employee.
Well, there is an outfit I've been seeing and since I'd been calling for several hours and getting nowhere, I gave it a try before breaking for lunch: https://www.zoro.com/
First contact: Gal who was far better prepared than the average bear; understood dimensions and thread differences, searched, suggested some close, but still shy of the cigar, asked me to hold for a bit.
(...de-dum, de-dum, de-dum, lunch is starting to call...)
Male voice (it's been on speaker phone) says "I'm David. I understand your looking for..." He also does a search, gets nowhere, but says "tell me how to contact you. I'll be back to you when I find something". That was yesterday a bit after noon.
I get an email this evening pointing out that combining 'this part with that one (instead of a single part) looks like it will do what you want'. Indeed, it will, and looking at the parts on the web suggests an even better alternative.
BTW, since this is the mock-up stage, the total order is in the range of $10 - $15; THAT'S service.