I’m no expert, but I looked at building a manifold for my v3 Reefer 350 before deciding that I’d finally build it when I need it. I ended up only hard plumbing my return pump and replacing the default drain valve with a gate valve (picbelow). Take everything here with a grain of salt. You may also know way more than I do . . .
I see you’re in the US, so a lot of your equipment will be imperial, not metric. I’d convert to imperial as close to the return bulkhead as possible. Imperial PVC and fittings will be easier to source and less expensive, too. There are lots of threads and YouTube videos that give examples of how to convert.
I got some good info on parts from sithrico’s YouTube videos. He is on this board but I’m having a hard time tagging him for some reason.
Here’s some info you may find helpful:
Return pvc is 25mm
Drain pvc is 32mm
Coralvue has metric couplers that are really useful
BRS carries some metric to us converters
Hth.
Took the easy way out here and used a bunch of Teflon tape and threaded a 3/4x1” reducing male adapter MPTxSocket. Available on pvcfittingsonline.