I have done most of the hacks considered in this thread, double cell, inductive charging (Qi and Touchstone) I have rebuilt the USB port and I have installed a secondary charging board inside a Mugen bulged case. I have not tried any 3D printed solutions despite doing that as aprt of my work as I have found that 3D printed thin shell objects are very fragile and I dont feel like getting into the effort of making a injection mold for a one-off and I have zero desire to do production.

-double cells in a Mugen cover tend to flex the board and I have lost 2 N900s to the no-SIM, I might try to reball them but my current N900 works fine

-Magnet cables are a cool solution but I end up with metal dust on me, it accumulates and I have had it attract to the magnetic attach cable and put on a small pyrotechnical show resulting in a melted cable end

-Qi(see my thread) add on kits can be cool if you are up to making the data short pins, also precludes OTG which I use. Of any except for the fragility of an unprotected ribbon cable it is not a bad solution, I might revisit it now that I have a Otter case but USB into a little Sharp Zaurus style drop-in USB charging base works for now. That and without the alignment magnets of the Touchstone system Qi is hit or miss for getting an overnight charge, I don't ever even use it on the devices I own which have it factory installed.

-Touchstone, this with a charge controller inside a Mugen double cell case worked pretty good for a N900 with a ripped free USB pad, the SM soldering is a PITA and airport security gets freaked out by the bomb phone.

-FDM 3D print of a Mugen cover clone, would be fragile and all the Acetone in the world is not enough, these parts need to be injection molded or formed sheet thermoplastic.