I usually make those plates with #12 AN o-ring fittings but I've also used 1 inch NPT and #16 AN o-ring.

The black anodized plates in the previous picture are a standard part number and pricing is on the webpage. The natural aluminum ones in the lower picture are a semi-custom part. The last pair I built was about $100 but it depends on how pretty a person wants them. I have a design that I think would work for most people but I just haven't built any parts for inventory yet. Shoot me a PM telling me what you need and I might have it on the shelf.