I think the best way to know for sure if this fits a 1200XL (other than trying it) is to measure the width of this cartridge, and then compare that to the measured width of one that does fit. I would do that, but presently and for the next few days I am away from my Atari's.
To this day it astounds me that this is still an issue. Maybe what needs to happen is to create a dimensional drawing of the 'ideal' cartridge size that will fit all, and then create a topic such as 'Atari 8-Bit Cartridge Dimensional Specifications' inclusive of this dimensional drawing. That drawing should also have dimensional tolerances clearly defined.
BTW, that is a fantastic looking cartridge you've designed
The 1200XL is designed to take cartridges the width of original 400/800 style Atari brand cartridges with some clearance.All cartridges made by Atari from the 800 to the XEGS are made the right size to fit the 1200XL. The problem is with 3rd party cartridges. Apparently Atari didn't bother to test 3rd party cartridges on the 1200XL or maybe they even did it on purpose. Only the width is an issue, the 1200XL's cartridge slot is plenty thick/tall. If it's that size or smaller it will fit. If it is wider it may not fit. I solved the problem myself permanently by using a Dremmel tool with a conical grind stone and shaved off half the width of the cartridge slot walls. Cartridges like the Super SDX cart by Dropcheck and my Gorf cartridge that did not fit before fit now, easily. I figured that was easier that worrying if a new cartridge would fit or destroying cartridge cases to make them fit. I don't have any cartridges using C64 cases, but I suspect they would work now too. All other Atari 8-bits have larger cartridge slots.
Edited by Gunstar, Sat Jan 7, 2017 7:59 PM.