Update: 28 Feb 2018 I've been going through my old designs and redoing them in Fusion 360 now that that is my design platform of choice. It's so much nicer working with a parametric modeler, even though SketchUp was quick and easy to learn. The dimensions on this version are slightly different than the SketchUp original, so the original is still available. As before, it's available without my initials and with. The first file is the one without as no doubt that's what most folks will want.
I looked at a bunch of the tire levers here - and there sure are a bunch - and printed several. There are some good, workable designs, but I'm pretty fussy about my tools, and none of them worked quite as well as my old workhorse, the Park Tool TL-4. So, I decided to make my own easily printable tire lever, using the TL-4 as inspiration.
One notable omission you may notice right away is the lack of a spoke hook like most of the other tire levers. I don't feel that those are necessary, and never use them when they're available, so mine doesn't have one. The lack of a spoke hook is part of why I like the TL-4, and why I was a little disappointed to see Park had reworked the TL-4 and the new TL-4.2 has spoke hooks. I also have no provision for these snapping together in any way - I see how portable tire levers might benefit from snapping together, but it's just not a feature I ever use.
The key bit of the design that I think makes the TL-4 style tire lever a better design is the relatively thin body, and the tighter hook at the very end of the lever which grabs either the tire bead or the edge of the rim and nothing more. I find the tight hook vs a long curve improves the leverage and precision that you can get from the tool.
I've got two versions uploaded, one with my initials, and one without. Obviously I like the one with my initials, but I won't be too hurt if you don't want that. Both versions have the "TL-4" designation as an homage to my favorite - now discontinued - tire lever.
My design shares nothing with the two levers that I listed as sources, but I liked their designs the most, and think they were the closest to what I was looking for, so wanted to give them some props.
I printed this both standing on edge and laying flat on the build surface. Although printing on edge seems less intuitive, my reason there was that the layers were oriented to best resist the bending forces. Laying flat makes for an easier print and no need for supports anywhere (although on edge doesn't really need supports, at least on my printer)
In my testing with a variety of tires, however, both prints seemed plenty strong enough. I didn't encounter any particularly tight tires, but look forward to trying these out on those.