Your best bet will be to link up with a vendor or LFS like Hightide and have them special order you some. As Michael said, most won't stock NPS corals as they take a lot of extra effort for us to keep and is a very small amount of people who actually will buy.
Also I wouldn't fold over the gasket how you did. It does not need to be as thick as the original as long as your bulkheads still have thread left to screw the nuts on fully.
Important part is that it sits square and flush with the tank. And you have proper gaskets on inside tank to flange, and...
Do you have a gasket on the outside between the bulkhead nuts and the overflow box? Also do not over tighten as this can cause them to leak too. Seated then just a tad bit more (sometimes even just a good hand tight does it). Over tightening can pinch the gasket and raise up a portion of it...
Do you have gaskets on the inside of tank for the bulk head flanges? You shouldn't need gaskets between the tank and the overflow unless you need to space the overflow box so it sits square due to the tanks rim getting in the way.