I've been doing this to copy large prints, and recently acquired a manfrotto arm from on here. It's like an old version of this:
Can be mounted on Supports with max load capacity greater than 6kg and 60mm disc diameter.
www.manfrotto.com
It helps, though you still have to weight the tripod or the other end of the arm to stop tipping.
Till I got that, my plan was keeping an eye out for a cheap Benbo tripod as they kinda are built with an arm like that, and often come up cheap as they're large and unwieldy. Also how are you currently mounting the camera? I flipped the central column, but with the tripod at maximum height and shoot down between the legs. The repro arm makes things a bit easier but doesn't massively increase the size I could copy. How big are you going/what lens? (for me, it also makes it easier to get over a desk to copy instax prints).
Another approach I took with some success, if you have a sit-stand desk, mount the camera on the edge facing down then you can move the desk up and down.
On the clamping the camera above, I'd assume the OP tethers to photograph like this - or if not I highly advise it, much easier to get framing etc perfect (and if you've not already seen the trick, lining up the camera perfectly square using a mirror makes it much faster to setup).