Here is how I made a strong and permanent hitch for my bike trailer.
The trailer is a Bellelli Eco Trailer Maxi
which I modified (with a little woodworking) to a luggage rack level hitch. Why? The original seat post hitch did not adjust high enough to ride level - and I hate seat post hitches, as they get in the way when mounting/dismounting (men's frame). http://www.bellelli.com/index.cfm?method=mys.dettprod&classe=14&id=40 http://www.mammacangura-na.com/eco_trailer_maxi.html
Also, the rack-mounted trailer hitch does not obstruct the rack (and space above), leaving it free for panniers and (er) luggage. When not towing, the eye bolt is a convenient and strong point to tie off luggage straps, bungees etc.