You're familiar with Autoslash I'm sure. You might take a look at their coupon site :
https://www.autoslash.com/coupons
Also, there's a relatively new ( I guess?) rental agency called Easirent that has a double shuttle from the car rental center ( as does Fox and several others). They look as if they're trying to compete/ get their rental car foot in the door in Las Vegas. I've never personally used them so have no details about service or other potentially important issues. You might peruse review sites, too; I looked at a couple of those and their rankings are not very good at all. But, the cars are cheaper .
https://us.easirent.com/car-rental-las-vegas-airport