I have the exact red/black fade sentinel myself. Awesome STBB marker. Probably the best blowback ever produced IMO. I am currently trying to sale mine
Best I could do is drill it through and tap the left side, so you would have to plug the right side. I would do one for you no charge.
If it isn't leaking with the larger carrier, then you were definitely using one that was too tight. You always want to make sure you are using the largest