Seriously, anything or nothing works fine.

If you're cheap, just lay a few cheap towels from Walmart or Target around the base of your bike, and you're good to go. I sweat ridiculous amounts even with a fan (I've poured over a cup of sweat out of my riser block several times) but a cheap towel is plenty good for sweat absorption, even for dudes like me.

I have a dedicated bike trainer mat for my Kickr, but I've been pretty underwhelmed - it's not like it does anything special, and is more for cosmetics than anything else. Unlike gym flooring where you are lying and walking or dropping weights on it, a bike trainer has you off the surface the entire time you're riding, so it's nearly irrelevant what's under your bike as long as it's stable and not super delicate to small amounts of sweat/water. And even with the special trainer pad, you will STILL have to use a towel to sop up the sweat pile if you're keeping things neat, so no real work or laundry saved.

https://kurtkinetic.com/products/trainer-floor-mat/

http://m.harborfreight.com/...-roll-mat-61241.html

I got this for like 9 bucks. Works fine and looks good enough for my bike set up in my office.