I messed up removing an evap cooler

Came out good.
I laid down a good bead of 3M 5200 fast cure and installed it.
Just to be sure I also used the 5200 around the plate edge.

I would have installed the fantastic, and then oil base sealed any exposed, then Eternabond over it all.

I ended up cutting a 4.5" x 17" piece of flashing and gluing it down.

Eternabond.

Eternabond.

I would cover it with Eternabond, not putty and goop.

You get a flat 3/16 aluminum plate just slightly larger than the imprint of the exposed area. You then use Dicor rood sealant and liberally seal the inner edge of the flat plate and use either rivets or flat head screws to anchor the plate. Make sure you cover the existing hole from the inside to prevent Dicor from oozing out and falling down into the interior. The Dicor and screws will glue the plate securely and will not leak and no need for Eternabond. Then from the inside drill 4-- -1/2 inch holes at each corner of the old wood square and then cut your 14 by 14 inch hole in the aluminum plate. You have a good repair that will support anything and you can attach Anything to that plate. If you want after the Fantastic install just use extra Dicor to cover the added plate and nobody will know what you did. Doug

Sounds like a good plan