This shows my folded trike in the car, immobilized by a two-piece anchoring system. The board with the cord is what anchored the trike to the floor. The short 1x2s screwed to the plywood restrained the chainwheel guard so that the trike could not move sideways, forwards, or backwards. The red cord tied the trike to the assembly, preventing up-and-down movement. The L-shaped wood next to one of the wheels prevented the wheels from twisting. With the trike firmly anchored I could stuff things next to it and under it. The set-up worked well.

Another photo is a close-up of the anchoring device.