a trailer - behind the Honda

[pekingduck]

The Sherp-X trailer looks pretty well thought out. It is well less than 70" from tip to tip, more like 65". It works because both ends of the trailer are suspended, so it does need for the rear-axle forward mount to pivot. It looks like a good design to base yours on.
The Sherp-X uses a 14" tire, same as the front of Honda's PCX150 scooter. I would much rather see that wheel because it is tubeless. Since most tire punctures are on the rear because of debris/nails kicked up by the front tire, that trailer tire has to fend off the debris from both tires. Get a flat back there and it may not be noticed until the tire and tube are well-shredded.

What I have in mind may sound weird, but I thought one could "over-suspend" a very light / small one-wheeler. Into the initial bounce (being kicked "up" by some hump) it would stay above the ground (say 1-2 inches) and slowly come down to the level position within ... say ... 2 secs. When crossing a hole in the asphalt, same thing, it stays up long enough to cross most dips. Such damping would obviously not work in a car or on your front wheel, but here it might ??? Of course this means that the trailer cannot freely pivot around the transverse axis, and that the mounting bracket on the bike must be able to handle the resulting torque.
Don't scream 'nonsense' ... I am thinking loud.

If the vertical motion pivot (for up-down motion) were mounted on the frame rather than the rear axle, you would have, effectively, another swingarm - one that pivots from above the rear tire. And you are effectively using a shock with little or no compression dampening, a lot of rebound dampening, and no spring. You'd still need a lateral pivot though for side-to side turning. That's a good Physics experiment......