Thanks for the info Steve
I have an angle grinder and a diamond blade--would that be sufficient?
For the urethane, do you mean something like this?
Tremco 10.1-oz. Buff Vulkem 116 Polyurethane Sealant-7103012 - The Home Depot
And to cut the metal on the bottom of the interior box wall (if I need to), would the diamond or carbide blade work? Does this type of blade work?
Dremel 3 in. Steel Metal Cutoff Wheels (3-Pack)-SM510C - The Home Depot