Hello my friend.
Don't know about affecting performance, as this isn't I assume part of a gas sealed component, only an inspection hole, however everything has a reason to be there, so if it is a naked flame or an exposed area of the unit, it might be a safety aspect.
I don't know what the part looks like, but have you tried making one out of an old piece of perspex ?.
I checked out some Electrolux parts dealers on line, it may be worth giving them a call.
Please try the following companies.
www.espares.co.uk Telephone : 0845-2306-230
www.leisureshopdirect.com Telephone : 0845-4300325.
Make sure you write the serial number down, and any other details on the machine and have that handy if you call.
Good luck
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