Brake booster to firewall gasket
 

Is there any reason I should order the gasket rather than using some RTV or a cereal box (I know there are some people around here old enough or cheap enough to have done this.)

Cereal box type will absorb & hold moisture between firewall and booster over time.
Silicone does a similar thing but has acetic acid in it, and will cause similar problems down the line.

Use a sheet of proper gasket material or use polyurethane body bonding cement ('Sikaflex').

(Yes I've used cereal box method of gasketing back when I was 16 & poor as dirt... never ended well in the long term.)

Do it once, do it right. This isn't something that's easy to get to, I would get the correct part and take care of it so you're not back in there down the road.

That gasket can't be THAT expensive.

Dale

Agree. Just buy it from Malloy Mazda.