Does the Orbtronic battery fail to work because the contacts don't reach or something? If it works, just use it.

18650 cells have some variance in length and width due to the protection circuits they may or may not have. Here's a review showing the dimensions of a bunch of common cells. You can probably find something close to the original battery: Battery test-review 18650 summary

If the original is protected, you'll want a replacement that is as well. (For more info, look here: The Anatomy of a Protected Battery) Obviously, the Orbtronic battery is protected, so this just applies to picking a different cell.