For me it is easier to create a rim for the existing tire, because I grind the rim to size and during grinding I am able to test the fit at any time. When I glue/stitch a tire I can only do that once. This is the tubular tire I selected with the DT Swiss 533d rim:
You have only 5mm height for the seam comprised of 2x2 stitched wires in case you use a clincher tire.
I ordered a 28" 24h tubular rim which would have 36holes@42". But as the tubular tire diameter is small, it will ride like a 36" uni. Today I received the shoe glue to glue the tubular tire. In former times I created a spoke thread roller. Now I bought 2mm x 1m steel tig welding rod to create any size of spokes.
Tubular rims have the drawback of less system weight and they should be welded together.
But this all is a side project with uncertain outcome.