You could use one of these:



All you need is a 2 inch square tube for it to plug into, a competent welder can fab up something that clamps to your bikes frame and is easily removeable. If you're going to use one of the big 4 bike jobs I'd recommend adding a wheel or caster to the bottom for support. Hell, it's probably a good idea for the 3 bike rack too. You don't want to lift the front of your bike off the ground.