Rubber Triangle On Radiator Cover For Thunderbird Storm

Hello everyone.

I have few questions about one little thing, that I have managed to lost, somehow...
I'm talking about that little rubber triangle on my radiator cover.

I know what it's for - to stop the dirt getting in the radiator, but after riding quite few miles in the rain, my radiator was still quite dirty around it, so I guess it was too small. But anyway - it seems I have lost it, so now I ask you for advice:

1. Do I need it?
2. Can I put a bigger one on, or will this compromise cooling ability of the radiator?
3. Does it have to be a special material or any will do?
4. What's the best way to attach it, as the glue that was used clearly didn't last?

Thank you in advance for any help.
Happy riding to you all.

Marcin