Instructables has an article about a DIY project to build your own "Xtracycle" type bike with an extended wheelbase for more cargo-carrying capacity. Though you need to sacrifice one full-suspension mountain bike frame and drill a couple of holes, no welding is required and the project seems fairly simple if you have the right parts. It involves connecting parts of a "donor" frame into the rear triangle of your bike to create your extended wheelbase bike. The main part of the bike can easily revert to its original identity, too. An actual platform to make use of the extra room is To Be Done in a future episode.
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I spotted a homebrew Xtracycle-style bike in Minneapolis a few days ago. It appears to have started with a donor frame and the entire rear structure is carefully welded from tubular steel. It uses a gear hub and disc brakes, and there is even a bottle opener mounted to the back.