So, found a couple of pieces of square bar, lopped them to size with the grinder, zapped them into place with the Mig and job's done. Now I have an air filter that seals all the way round, and the lid is nice and firmly held in place as the spring clips actually have rubber all the way round to compress.
![Image](http://img.photobucket.com/albums/v418/typhoonpud/Alfa/Alfamods004.jpg)
Regards, Andrew.