To improvise given your current predicament, you could perhaps add a outer ring of polyethylene or other impermeable materials around the membrane, say 0.5-1 cm thick, and then keep this consistent across all samples tested.
No I have not test them yet . Because we do not have it and for the tests we should prepare all of the samples first and then send them some where else that's why I am not sure to continue with this size
Even in some cases it is becoming smaller in size and I could not correct it yet ..
are the rings usable in any desired size ? i mean for example some times my membrane are 3 cm ! in diameter is it also possible for this sample to use the ring and do the test in the cell or no?