Tom, your pictures gave me a good idea regarding the addition of d-rings. While doing some research, I found about Anchortrax. Wells Cargo seem to be using these on their trailers. I will explore more about this.

After some careful design using sketchup, and thinking it all through, I decided against building it myself (time constraints and a desire for a top notch end-result) so I went today to the manufacturer and discussed some of the options.

I decided on it to be 12 feet long, 5 1/2 wide, and 6 feet tall. all these are interior measurements, and will all accommodate large wingspans of up to 140".

The base will be iron, painted and treated for corrosion. The rest will be aluminum profiles with aluminum siding that already includes oven cured paint. All edges are sealed with silicone. Will have flat spring suspension on 15 inch wheels, single axle, and I will add one caster wheel jack, and on the back, a couple of stabilizer jacks, so that I can leave the trailer unhitched and be able to work inside with no fear of a tilting trailer.

Side opening back doors and a side door measuring 27 inches wide instead of the standard 23 or 24 they use here.

All doors will have a proper seal so that way no dust comes in for long storage periods.

What I am still thinking about is how to run additional electricity connections.
I basically need 12 V for charging, maybe using a deep cycle 12V, to which I can hook up my chargers, inverter, etc. I still need to figure out exactly how many amps and how large an inverter, and of course some sort of transformer to convert 110 ac to 12VDC so I can plug it in the night before flying to charge the babies.

Photos, plans and details to come soon

Tips appreciated.