You shouldn't need a large seal on the part that mates to the manifold as the manifold already has it's own O Ring present - you'd only need a seal there if the bypass was poorly made and as such didn't seal well.
I exclude those items to keep cost down, but if it's something people would want I'd of course consider adding them.
Quick note - although my PayPal sometimes shows as Magical Solutions, Magic Parts aren't related to me in any way. I remember one customer wouldn't believe me, and started complaining about them to me!
