So, does anyone have a source for the material sandwiched between the 2 plates that make up the valve in the front fan assembly?

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914Rubber has them. They have nice kits for complete air box refresh.

https://shop.914rubber.com/914-Airbox-rebui...m?categoryId=-1

Here’s the one without the grill. It includes the large seal, the flapper seals and new top seal.

Also have grills and improved grill seal.

Also the grill seal is molded
Cheapest to add to the grill to the seal if you want both.

https://shop.914rubber.com/914-Fresh-air-box-top-seal-SEAL-ONLY-Late-for-use-with-grill-914FAS.htm?categoryId=-1

I've just finished my air box with 914Rubber parts. They fit perfectly.

Nice,
Would you mind if I use your picture, it's better than mine