Depending on what motor you have, if it’s the type of gasket that sits on the lower cowel and is not visible, I just take old fuel hose leftovers (like the 1-3 inch pieces of extra trimmed off) and make a slit then place them in 2-6 spots. They are fresh rubber, you can’t see them, and they are basically free (usually thrown out otherwise). You only need a 1 inch strip in a few spots to really make the top cowel latch down tightly.
None of my customers seem to care either because most almost never go into the engine compartment anyway.