Yup, this is the exact issue I'm running into and it's frustrating because it seems like in order to do this (multiple stacked monitors), I have to do this... (replaces desk).
The lip on my desk isn't deep enough to support a clamp, and honestly it doesn't even feel super stable with two monitors on bases on it... so now i"m desk shopping.
I currently have two Dell 27 inch monitors side by side (S2721Q) and then a third vertical monitor off to the side for Slack, all connected to a GEFORCE GTX 1080. I like to do 2x2 stacked (4 total), sticking with the S2721Q's + the vertical. I could go smaller on the top monitors as I don't need as much screen real estate for those, but I feel like a smaller size is going to look really funky.
However naturally the secondary issue is the GEFORCE can only support 4 displays, so I'd then need to try and figure out how to run the 4 stacked on the video card and then try and do the 5th display via the on-board integrated video which seems like it should be easy enough, but never is.
But for now, I'm desk shopping. I don't need that much actual desk space for writing things out, but need it strong enough to support this setup.