[notout86]

If you think about it, I believe you will realise that it is misguided to ensure the tank is sealed.

It should be open to the atmosphere to allow air displacement as coolant expands into the overflo tank and air goes back in as coolant in the radiator contracts as the system cools.
If the bottle/tank was completely sealed it would be bulging and collapsing. It is not flexible like a bladder. THERE IS A BREATHER HOLE AT THE TOP OF THE AIR SPACE but it is concealed under the metalwork. If the car tips over coolant will come out. A flexible bladder would be the ultimate.

In my opinion that hole is too large and allows dirt (and possibly insects) in & if you open the bottle cap and look inside you will see dirt below it. A bit of open-cell foam to act as a filter over the hole is a good idea. The hole should NOT be blocked/sealed.

Please correct me if I am wrong.