sorry, but if you have a squirrel, or other type of dead animal in your attic, next to the venting system, the only way to really fix it is to call a company to come get it, OR send a big strong nephew, or brother in law up thar. Pay um well in beer or cooking.

As for kitty boxes, you can mix baking soda in with the litter, and that really helps keep down on the odor. My baby kitty tend to hang her fanny over the edge of the box, and drop presents on the ground beside the box, the lil monkey! so,be sure to pull the box OUT, and check around it or UNDER it to be sure urine didnt dribble down the edge and under the box.

they sell stick up type products that you can open a vent and slap it into the duct work till someone can crawl up there for you. Just make sure that you buy all from the same family. if you get fruity, use all fruity, or all flowers, dont mix them up, or the mess of perfume odors can be just as bad as the dead critter. You can also stuff dryer sheets in and around the radiators, or the vents. Cheap and cheerful. Dryer sheets are amazing! If you actually have a dead critter, make sure you send some one up there to stuff dryer sheets around the openings. I close up my RV with them, and the mice/rats hate the stuff, and will boycott my vehicle.

If worse comes to worse, make sure to strap on a sense of humor, and realize that sometimes critters do what critters do, and you will get to it when you can.