I use a few cardboard boxes taped together with gaffer's tape. The tape is waterproof. 2 layers of cardboard (staggered at edges if you're using small boxes) is plenty to make it through a season. Use the box edges up against a wall to prevent splatter. If you have uneven floors and the water rolls off of the boxes, fold an edge over to cause it to be shimmed up a bit, keeping the water on top of the cardboard.
My heavily used cardboard is due for replacing now that we've had quite a bit of snow. No leakage on the hardwood underneath it though, for a few years now.
If I could find one again... I used to have a plastic shipping pallet that was rectangular with a solid bottom and a decent lip on it (3" or so). I would put cardboard inside (like those cat cardboard circle things) to soak up water, and then the bike on top. That way there is a guaranteed solid base, and you can still throw away the cardboard layer taking all the road gunk with it. This might work with Tom's idea if you can find a tray that's deep enough.