I can see 2 possible problems (1) the opening is to big should be as close as the size of the a set 110 and (2) the slot your spring slides I'd is to big should only be about a half inch , probably (2) more than anything, jmo
Try placing a small piece of wood across the bottom of the cubbie that fits snug between the jaws. Kinda like this except on the bottom of the box.
Or try something else to stabilize the lower jaws, killer clip or a piece of tin, hose...
You are probably not going to have a high success rate with a 110...
Red squirrel? I would shoot every one of them outside, seal their entry and line with tin. And maybe use rat poison with a few peanuts mixed in for good measure. Red squirrels can be very destructive, I would hit them with everything and git-r-done!
Borrow a 160 size BG (6 X 6) and make a small cylinder wire basket out of 1/4" mesh about as big around as a 0.25 piece and 2" high. Pre-bait if you want but try corn or sunflower seeds in your bait basket. Fasten basket to trigger wires centered on trap jaws AND since you are inside you can eliminate the cubby as the trap is large enough make a catch from any direction. Any type of BG holder/stabilizer fixed to a 2 X 4 will suffice in your attic. Squirrels are pretty good with their hands and will try like heck to get at that bait. The 160 size fits about any size squirrel for me.
1st,don't put bait on the trigger!You have the trigger,closest to the animal and when he grabs and pulls,he is or may be out of the trap when it goes off.That is why the trap is pulled toward the entrance.Put all bait in the back of the box,behind the trap.Open the trigger a bit.
I have boxes like that but I have 2 small blocks attached to the side,opposite the slot.The hinged jaws go on each side of those blocks and keep that end from moving.If I think the spring may move,wedge a stick in there but not overly tight.Still,I will not put bait on the trigger.A small amount can be scattered outside as an enticement.
I like peanuts, individual corn seeds and peanut butter for bait. Put 1/2-fingernail sized or dollops of bait in front of the cage trap and just inside, leading to more bait behind the pan. This is a Ron Jones idea. I canít take credit for it. Anyway, my only squirrel sized cage trap is a small dbl doored Havahart. I only have one door open when set.