I was not able to find the rubber one from Rawalpindi, so had to custom order a plastic-**** one; which leaked in 4 months. Then bought a new gasket from Jabbar Autos, Gawal Mandi, Rawalpindi made of cardboard sort of material (gatta). It is leaking very slightly. I am now waiting for the leak to subside itself in a week or I will apply some silicone sealant (found a slightly expensive one, looks good, says on it that it was made in USA) till I find this rubber one.
But the transmission chamber should not be opened regularly as it increases the chances of warping the chamber (causing leak) or introducing impurities in chamber.