After planing a piece of Brazilian Cherry left over from the construction of my dresser, I made two "keepers" to prevent the mirror box from sliding from side to side when the telescope is pointed near the horizon. After planing, I ripped a piece one and a half inches wide, then cut two pieces six inches long. I then bobbed the top corner and drilled two holes then countersunk them after reducing their width to 1 and 1/4-inches. Two holes 5/16ths of an inch were drilled all the way through for the 1/4-inch machine screws to pass through. On the rocker box, I made two pairs of blind 3/8-inch diameter holes to accept threaded inserts, which were installed with a bolt and nut after dabbing epoxy on them to keep them from coming back out. The keepers were varnished with three coats polyurethane and before finishing I notched out the sides of the tops facing the altitude bearings to prevent them and the keepers from scuffing each other.
I've also begun to order the components necessary to complete the telescope. I've already ordered the secondary mirror and a holder for it, and soon I will purchase a focuser and a finder telescope. I am planning to also install a Telrad. While I made the assumption the upper cage will be quite heavy for a 15-inch Dobsonian, I can add strips of steel to the tailgate if the telescopes turns out to be top-heavy.