The best one for that (IMHO, though I should note this company sent me one to test, and it's been running my NVR for months now) is the Axzez Interceptor: https://www.axzez.com/product-page/interceptor-carrier-board

It's purpose-built for that, and they even have a rackmount chassis for it, though you could design your own too. Power for the drives requires a separate power supply though.

The other option right now is the Radxa Taco (https://radxa.com/products/io-board/taco/), but availability has been scarce. I've ordered one and will hopefully test it again soon, with the CM5!

(That is, if you're wanting to go down the Pi NAS route—there are certainly other options out there!)

I have one of these (the axzez board) with two hard drives in a cheap supermicro 1U case, works fine

Those both look good, though I might have to save up a bit to afford them haha. I'd probably end up printing my own case, so that's not really an issue. Looking forward to your testing of the CM5 Taco =)

I have the Taco and was about to ask whether you have tested it with the CM5 yet.

For some reason I thought that Radxa is about to deprecate the Taco but I might be mistaken.

The one issue I found that without a heat-sink, fan and any drives just idling on a 12V supply the board got really hot. So I have my questions about the efficiency of the board. I've been thinking about testing the power usage of it so maybe a good time to follow through on it.