As above, the metal tabs can be bent outwards to make it fit tighter. So you could make the plastic cap fit just as tightly probably.
The rubber gasket ring can wear out, I replaced mine with an aftermarket part and it kept leaking but I then got a genuine Volvo one and it's now fine. So you may want to look into that.
Also to stop further leaks elsewhere, you should probably replace the oil trap box (below the manifold) even though you did the flame trap, if your car has EFI just take the throttle body off and you have enough access. (Tip - use only genuine Volvo part, and do an oil change straight afterwards) It's an important part of the PCV system; compare the brand new one to old one with 400,000km: