We do that all the time on a as needed basis. Don't have any specs or plans though. We usually take a hitch that has the 1 1/4" receiver, cut it off, and weld a 2" box on. People usually do it to accomodate bike racks and things. If you are doing it to haul a heavier load, make sure the structure of the hitch and car can take it. We caution people who are doing it even for bike racks that they may still cause damage if overloaded.
There are adapters available that go into the smaller receivers and expand to the 2" one.
