This is a minimalist child seat designed to fit a specific bicycle frame, the Fetish Obsession. The seat mounts to the top tube of the frame and the footpegs mount to the down tube. My copilot then sits right in front of me, holding on to the inner part of the handlebars. He loves riding around on this seat, I think he feels like he's actually driving the bike.
But isn't that unsafe? Not really, he's directly in front of me where I can keep an eye on him and he's nestled in between my arms (and knees since I rarely pedal this electric bike ;) where he can't go anywhere.
There's a picture in the gallery of the full bike, but this was when I was running a steel version of this set-up- I included it to give you an idea of where the seat and footpegs go. The footpeg mount uses a couple of short sections of 3/4" PVC pipe (schedule 40) I had lying around. I glued the PVC pegs in to the mount, but the mounting bolt (M6) which holds the mount to the frame passes through the peg as well- just drill a hole in the PVC when it's mounted in the footpeg mount using the mounting bolt hole as a guide.
I doubt this will fit any other bike frame without modification, but feel free to modify the included sketchup files.
I forget the actual infill, but I'm sure it was in the 70-80% range. I also acetone dipped the footpeg mount to increase the strength. So far, everything is holding up nicely.