You can find these cheaply from the usual sources like AliExpress and eBay. The mounting for these are awful, they put the mounting holes essentially in the edge of the board. I was able to get the dimensions on this pretty close to the actual board without having a mechanical drawing. The way this works is you insert the USB connector at an angle and then swing the board down onto the little posts. Then install the two hold downs with screws. You can then mount it anywhere you want with wood screws or whatever fastener is appropriate for you. Each side has a series of loops meant to allow you to use a small zip tie to hold down any wires you might have connected to the relay.

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usb_8ch_relay_mount_base.stl

usb_8ch_relay_mount_hold_down.stl