Router sled...

[Ljohnsaw]

I have one of those centipede units, a 9 pole unit and will be trying it this week.

For milk crates, I'd stack them 2 or 3 high.

Yes, those wheels will work great. They are a bit small at a half inch in diameter. Mine are about a 1.25" and I think roll better.

[rusticretreater]

I am not keen on using aluminum for things like this.  Aluminum has a higher wear rate and is susceptible to corrosion over time too.  Steel or metal wheels on the router sled are harder than the aluminum, so all the wear would be on the aluminum rails.