OK, I had some time this morning, and I did some more mocking up.

Some pics....
















Just to show that the clamps fit very well on the tubes

Compared to the first mock up, in this one I moved the bottom clamp up the seat post a lot higher. I did this to get the clamp on the part of the tubing that is not butted, as in the thicker part of the tubing. With the seatpost out I could look down into the tube and you can see where the grease from the seatpost is not smooth, this is the area, I assume that the butting starts as the tube inside diameter increases and the seat post no longer touches the inside of the tube.



If you look at the above pic, I've marked a few things out.

In red is the height of the trailer's tongue, I want the hitch slightly below this height so I can adjust it upwards to get the trailer level with washers. In the picture it looks well below that level, but is is not, that is just the camera angle.

In green is where the internal tube butting starts. I put the clamp above this level by a bit, just to be safe.

In Yellow is where the seat post ends. I was surprised at how long the seat post is, but there you go.

I think that by doubling the thickness of the clamps, and by using this design, it should work very well, be compact and out of the way and a LOT lighter than what I have on there now.

Comments, suggestions, questions etc all welcome!

Cheers!