So the imperial parts are sized for a literal 1 1/4" rod which is not how PVC is sized unfortunately. So 1 1/4" PVC pipe has a larger OD when adding in the wall thickness. I’ve been using 1 1/4" wood dowel rods which fits correctly. You might also be able use EMT conduit though as that in theory should be a “tube” and is sized by it’s OD.
Lastly, if you use wood, you just need to re-design how the vertical rod terminates into the horizontal gear box since you can’t insert the azimuth axis pillar into the vertical pipe. You just need to drill a centered hole .5" deep in the vertical dowel rod and insert a ~1" long by 5/16" steel rod which can fit into the bearing center. I just cut a piece off of a 5/16" rod from a scrap printer.
I’m thinking there is a set of parts resized for actual imperial PVC size floating around, but I can’t seem to find them at the moment.