Kaz:
If you're talking about one of the front-wheel holders -- that you plug into the receiver -- all it takes is some channel, some tube, a few odd pieces to brace, and so on -- weld up to fit your needs. Make sure you measure the height from receiver to ground correctly when building, and when you make that measurement -- figure that your tow vehicle will be fairly well loaded and therefore low, and also include the weight of the bike. If not -- you run the chance of plowing a furrow down the return road as you pull the bike. As always -- DON'T ASK ME HOW I KNOW!.
And also keep in mind the problem that you'll have if the outriggers that hold the front of the bike don't allow for turning -- or you'll hurt something, like the afore-mentioned steering stops.
It's because of these things, and a few other issues, too, that we've gone to a wheeled tow-back rig. This works way better -- and can be bolted to the floor of the trailer, the bike strapped to it -- and then off we go cross-country hauling the bike on the hauler in the hauler. Good Luck.
