Foam stopper when refrigerating a starter?

[dotJerdot]

I currently have my starter on a stir plate with a foam stopper from NB and would like to chill it in the fridge to decant.

Should I change the foam stopper to an airlock to prevent any type of contamination from the fridge or just leave it with the foam stopper?

 

[Haasobrew]

Foam is fine

 

[badbrew]

I'd use foil but it shouldn't matter much.