Regular pipe flanges can be had with the usual pipe threads to fit threaded pipe (same as rigid conduit).
You could then drill normal camera mounting boxes and bolt them to the flanges.
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Around here, you can get pipe flanges at Home Depot, Menards, etc. Same with pre-threaded lengths of pipe and counduit.
You can get flanges in iron or plastic. You could even use plastic schedule 80 pipe with glue adapters or thread it, etc. There are a lot of possibilities depending on the materials you prefer to use.