Element Pikes is a clone of a famous trifold bicycle. However, one design oversight is the seatpost doesn't lock precisely with the frame when collapsed, so unlike the famous original, this bicycle can't be carried without unfolding. Unless you add this part!

Print the STL in its native orientation (flat side down) with supports for the two round jaws. Push the jaws into a matching circular slot on the bike. The jaws are oversized by 0.2mm so it should be a tight friction fit.

For strength, the jaws should be fully solid. I use 6 wall lines and 50% infill.

Parametric design in OnShape: link