I agree with Paul that the outside is mostlybetter but sometimes wires or other dangers will drive me inside. Sometimes entering the attic stresses the animal and it might take a couple of days for the squirrel to get comfortable with you trap. Cage traps work inside but then you have a customer that may worry about the squirrels comfort. I like to put the conibear over the hole like you tried, wooden cubbies with conibears, tube traps and cages on the roof.
corn on the cob, peanut butter with black walnut oil or black walnuts are some of my favorites.
If you are doing the job at a flat rate and you don't need a head count I would use vanish from WCS. Raccoons prey on squirrels particularly babies and I believe I have moved some with it. I'm getting ready to be more scientific with my research.
Sleep'n with an animal..I can help.
Do not use both feet when testing the depth or temperature of the water