Hei Feliks!
I also really wanted to be able to use a Shotgun seat on my Clem, but I just couldn't figure out a good way to make it work. The one I have is frame mounted, so it's possible that a stem/seatpost mounted version could work (both Shotgun & Macride make these), but that was a non-starter for me since I don't think I could even straddle my 64cm if it had an actual top tube...
So my solution has been to keep my old MTB running with the Shotgun permanently mounted and ready - it's still by far the most fun way to ride with kids between the ages of 2 & 4, even if you don't get to be on a Riv!
Pictures of my futility: