Gravity Based Surface Contact Wheel

Using multiple models (2 not including the wheel) you can mount a wheel to a 0.75 inch x 0.75 inch piece of t-slot extrusion. The wheel will always be in contact with whatever surface it's above unless the surface is super uneven, in which case you can simply change the model for your needs and purposes. 

The black front bracket slides independently of the white back bracket because they're loosely connected using nuts and bolts. This allows the front to slide, but only as far as the slot for the bolts allows. 

The holes that are used to mount this contraption to the extrusion are small because they are simply guide holes. You will need to drill or change the hole diameter to whatever width your channel nuts are. You should also glue down or use an adhesive because the channel nuts only keep this contraption aligned with the t-slot extrusion. 

You can easily find the wheel used in this model online or something similar, but I also made the wheel in CAD so you can download it as well. 

There's a good chance you'll need to sand some things down, because the tolerances are very low in order to make the parts fit perfectly. The holes will also probably need to be changed depending on what nuts and bolt size you use.