Look through this album of mine on photobucket
http://s182.photobucket.com/albums/x314 ... 20trailer/ it should show you how I built my tow bar. I did mine different to most people as I didn't want to drill holes into my floor so I made it bolt to my subframe only (a total of 16 bolts).
The pics don't show it clearly but I only bolted through the sections of the subframe that are double thickness and I used some plate/washers on the top sides to spread the load over an inch or so per bolt. I have also strengthened the rear crossmember of the subframe by welding in some box section from left to right (I did that years ago when the subframe was out of the car so I could jack up the center of the rear with no concerns about bending that crossmember or mucking around with blocks of timber).
It's all still solid after 5 years of use. It may be heavier than the std design but I've seen many rusty boot floors and rear seat bases and even if their nor rusty I don't want a trailer hanging off my boot floor. The way I see it is you should be confident to jack the rear of the car completely off the ground by the tow ball, if the mounts flex or can't support the weight of the car do you want a loaded trailer hanging off it?
