I never liked the bulky shape of the TI-58/9. It was necessary for the 59's card reader but not for the 58. Opening it up shows a lot of empty space. So why not cut it out?
A kind forum member sent me some disused case backs and I applied a Dremel cutter and some wet sandpaper. I added a new clear bottom and a cell phone battery. It needs finishing, such as black paint around the edges. It's now only as thick as a TI-84 and I hope to use it as a daily driver, if the battery life allows.