I was able to get under the car this morning and I saw that it would be extremely easy to build a hitch. Just some square tube and channel, about 4m, and 3 or 4 U bolts. I am going to ask my friend if his welder friend can help me out. I might be able to do this for only $100. If only I still had a membership to
www.techshop.ws I was also thinking about mounting a cheap universal hitch where the spare tire is located. There are some real simple bolt on hitches that I think I could distribute the weight enough for it to not tear out. I am getting a second opinion on this tonight. This option might only cost about $50.
I saw the post you linked to, but I was wondering about saving a nice chunk of change. If you look at the installation instructions for the hitch, it just bolts into the same metal I was refering to. That's what the bumper supports are hanging on. Depending on what exactly needs to be done to the z3 hitch, it might be a good choice so I don't have to drill many holes. I wish the guy from the post would have posted pictures of the modification.