Here's a project I've been working on over the winter. It's come along enough that you'll be able to see where I'm going with it.
I took an old girl's frame mountain bike and cut out the upper down tube.
This weakened the frame, of course, to the point that I wouldn't even dare to ride it.
But maybe with some reinforcement.......
frame mods 01.jpg
frame mods 02.jpg
frame mods 03.jpg
I'm not able to estimate whether or not this is strong enough for the job. My gut feeling is that it is. When I get the bike together the way I want it, then I'll just test ride it carefully for quite some time and keep an eye out for looseness.
If all goes well, then I intend to build something that will be reminiscent of one of these;
Though I can already see that the gas tank up by the stem is not likely to work. There's very little room. It might have to go over the rear rack.