We have the same type of slide-out bike carrier and have found that it is not really designed to easily and safely fasten bikes to it. In a word (or two) it is quite "Mickey Mouse". You need straps, bungee cords and whatever else to keep the bikes secure.
Instead, I would recommend that you weld a receiver hitch to the frame, purchase a good quality bike rack that fits a standard hitch and leave the slide-out carrier retracted. You'll save yourself time, avoid frustration
, and it will be easier on the bikes as well.