Ideally you'd want an enclosure or docking station of some sort. For a 3.5" HDD you need something that has its own power supply. Something like this or this (that 2nd one seems too good to be true for only $10, but who knows).

Hmm I'm not sure that that will work with a standard 3.5" HDD. Power is going to be an issue. That's expecting you to provide power via the USB plug, and the product listing says "If you use it with your 3.5 or 5.5" HDD, then please make sure that your USB devices offer 12V Power." The USB ports on the NVR certainly won't provide that much. So you'd have to find a USB power block that supplies 12v. Most are 5v I believe.

Seems like trying to use that is using it for something it's not meant for.

Most people use an enclosure like this: https://www.amazon.com/StarTech-com-eSATA-Enclosure-Trayless-S351BMU33ET/dp/B076M3ZLKF

EDIT: Or this one, which is one of the cheapest ones I've seen: https://www.amazon.com/Vantec-NexStar-Gen1x1-Enclosure-NST-358SU3-BK/dp/B0BDKG16XT

Depends on where you are, but in the US there aren't many available anymore. The only 2 I can ever find on Amazon are these 2:

https://www.amazon.com/Vantec-NexStar-Gen1x1-Enclosure-NST-358SU3-BK/dp/B0BDKG16XT

https://www.amazon.com/StarTech-com-eSATA-Enclosure-Trayless-S351BMU33ET/dp/B076M3ZLKF

I have this tray style one, but it doesn't seem to be sold anywhere anymore.

You would just want to buy an eSata enclosure or dock like these ones below, and then put the HDD of your choosing in it. eSata enclosures aren't that common these days, but there are usually a few to be found.

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https://www.amazon.com/Vantec-NexStar-Gen1x1-Enclosure-NST-358SU3-BK/dp/B0BDKG16XT/

https://www.amazon.com/StarTech-com-eSATA-Enclosure-Trayless-S351BMU33ET/dp/B076M3ZLKF/

https://www.amazon.com/StarTech-com-eSATA-Drive-Docking-Station/dp/B00KILQOG0/