Some random thoughts for your project
It is by far best to use a hole saw insead of a saber saw.
Start cutting from inside so you don't splinter the wood headliner
. Once the pilot bit penetrates the deck
, take the drill outside and use the pilot hole to finish the 4" hole from the outside. This will give you nice clean holes inside and out.
After you have the hole cut out, rout out the deck
coring for 1/2 to 1 inch back from the edge of the hole. With a plywood
cored deck I'd probably use a dremel tool to do this, but good sharp chisels would work if you are careful. Replace the plywood removed with highly thickened epoxy
. This will keep water
from contaminating your core
and causing you no end of trouble.
I think those are the key points. Somebody else might have something to add...