TRIM LOK INC Rubber Seal,Tubing,0.28 In W,25 Ft - Foam Rubber Seals - 10G529|X311-25 - Grainger Industrial Supply

This soft .28" foam tubing arrived the other day and lays nicely into the half round depression molded into the rear of my header tank.

I secured the tube to the tank sides, to about 10 inches up with a few dabs of hi temp silicone and secured the bitter ends with one loop of lacing tape.

This might just be the answer to a perfect air/water seal.