Hey Probtrapper, I see you're in CA too? You're dealing with the same thing-or Belding's squirrels? Nasty, destructive, germy little buggers. We're in plague country down here...dunno about up there-but use PPE...it's not worth getting sick.
I use repeating traps...they are 24" x 24" and have two center doors, directly opposite of each other. No way I could use single traps...I'd literally need thousands of them.
I bait based on what time of year it is. Mid spring when things start to dry out here, cheap, greasy dog food and lure. Lure at all times.
If the dog food smells like bacon, even better. By summer I've switched to a combo of the same cheap dog food, but add dry corn. 50 pound bags at the feed store are around $13.00, and also add black sunflower. I went through prolly 600 pounds of dog food, 100 pounds of dry corn, and 25 pounds of black sunflower this season. They seem to go for those items until late summer, and then they're getting real thirsty, so any non-citrus, old fruit (grapes, apples, melon slices, etc. all work well...but I still keep the dog food/corn/sunflower going.
Using a lot of bait for big infestations is a necessity, as more squirrels enter, they just start eating and stuffing their cheeks.
I get my traps from a trap maker up in the Redding area. All hand made, no cheap, mass-made Chinese crap. You already know we have two choices, dispatch on site or release on site. Takes me maybe 3 minutes to dispatch a trap full of them...apparently we're not supposed to talk about dispatch methods I think...(I'm still kind of new here) so PM me and I'll tell you what I do. It's legal, cheap, and fast.
My record in one trap is 25...in 2 hours. My sets are always in the shade...something they're looking for when it starts to heat up.
I made rigid, cheap plastic covers that I can pop on the top of them with a rock..to keep them focused on going through the doors, and keeps ravens from trying to get bait (or at squirrels) from the top of the trap. I often use the same plastic (sheets of it at Lowes, Home Depot) for a few bucks, cut them 4" square larger than the trap's footprint...which keeps them from trying to dig under to obtain bait.
I'll edit my post when I find the link-his web site is sort of old...and I don't want to give you the wrong link!
These traps show a 1x2 mesh, but my newer traps are 1x1 mesh, and I like those a lot better. Keeps birds from stealing bait, and its too small for them to reach in far enough to steal bait.