It both seals the fluid from the air and allows for rising and falling fluid levels as the clutch is used, so cutting holes in it or using clingfilm isn't the answer.
Ian let's have a meet up as you suggested in your other thread. If you had a leaking slave cylinder eventually the fluid would run out, and as you have a sinking pedal with fluid still in the reservoir either air is getting in somewhere or there is a fault with the master cylinder.
You can always have a borrow of the GhettoTech clutch fluid level indicator if you are worried about it dropping.
