Have two holes in the base that you could clip an old guide in to or two slots to take a strip of plastic to act as a blade that will then fit in the slot of the track to keep the start unit in place. Stick copper tape, as used on routed tracks or on Scalex sport track, on to the start unit and run both ends under the base of the start unit so they make contact with the track rails. Depending on the weight of the start unit this may be enough to keep contact but as an insurance I would make two pockets in the base of the unit, may be at both ends, over the track rails and glue a traction magnet in to each pocket, these would pull the unit down on to the track. I would also fit the magnets before running the tape under the unit so you have track rail - copper tape - magnet. So long as you buy copper take with conductive adhesive you can build up the thickness of the tape under the start unit to improve the contact between tape and rail.