I'm thinking about something similar to transport my backhoe. I've not put anything on paper yet, but I've had the tape measure out and have ideas in my head. What I've got thus far would be made similar to a "tow dolly" used for cars; the type you drive the front wheels up on, strap down, and tow. I would have some suspension under it unlike a tow-dolly, and there would be room under my hoe for the suspension without issue. Basically though, it would just be an axle with springs hooked to some bits of framework with a somewhat long tongue.
How I see it working is to back it up under the backhoe, set the stabilizers down on framed up "stirrups" mounted directly behind the wheel/fender of the trailer, set the bucket down on a stirrup up on the tongue. Pull the pins and get it loose from the tractor but leave the hydraulics connected for a moment. Lower the hoe to the trailer frame with the stabilizers (they would need folded up or the fenders would need to support them the way things measure out on my hoe and a standard cheap axle I had laying around), there would be sections of channel frame the backhoe mainframe would lower into to rest. Tie the bucket down to the stirrup made for it up on the tongue, then use the backhoe arm curl to drag the mainframe of the backhoe a few inches further up onto the trailer above the axle and tie it down. Disconnect the tractor hydraulics and off you go.
Another thought I had rather than sliding the mainframe forward onto the axle of the trailer was to have a "hitch receiver" somehow mounted at the back of the trailer that I could slide a piece of subframe into for supporting the end of the hoe mainframe. That seemed too much like manual labor though. The hoe shouldn't have any issues dragging and pushing the hoe mainframe in the channels to shift the weight up onto the axle.