Ikea Frosta Bike Hack
This is a popular project designed by Andreas Bhend and Samuel Bernier. I decided to build one for my daughter. The source files are available here. The designers also wrote Ikea style assembly instructions. This project uses a combination of two FROSTA stools, 5mm style fasteners on 1/4" threaded rod, and 3d printed parts. The project comes together well, the FROSTA model stool has changed a bit - I noticed the legs taper at the end that attaches to the stool and are not longer exactly 90 degrees which creates a slight clearance problem for the rear wheel. I recommend you mock up the bike before you drill any holes for the fasteners or cut the seat parts F2 and F4.