Fred, this is what someone else posted in the forum, it might work in your situation.

I put up a couple big birdhouses with 3-inch holes. This attracts
owls and kestrels to the area, something which squirrels & chipmunks
don't find very amusing. The squirrels will use boxes with 2-2.5 inch
diameter holes, but around here they stay away from the 3-inch ones.
Birdhouses with 2-inch diameters must have a steel-plate around the
hole or else the squirrel will widen it to their own dimensions
(usually pregnancy dimensions, meaning more squirrels). Animals seem
to know that a hole that's too big is a sign something bigger can come
in after them. In fact, I've been working on building a barred owl
nestbox, but that is a biggie (2.5 feet deep and 12"x12" base with a
7" entrance hole). I would just barely have enough wood.

To be honest, the pellet gun is what works best. Usually I get 30-40
a year, and there's always just as many to fill the void. With a
squirrel it's less effective, but a properly sighted scope is very
effective with chipmunks.

Dan