OK.
Today was my "research pee prevention options" day during my lunch hour and a bit after work, and here's what I would suggest.
Maddog's solution is still the best, regarding the weatherstrip idea -- I just think he forgot to clarify some details.
If you're using any of the tray designs posted here at the side (or the Forever tray), the side skirt height of the tray should all be just about the same -- that is 2 1/8" from the floor, which means you end up with a very small gap (less than 1/16") between the tray and the SF.
If you tip the SF unit up, you'll notice an area (about 1/2" wide) where the tray meets the box (where that gap is). THAT is where I'd apply the self-adhesive weatherstripping that Maddog mentions. You don't have to get that exact part number, just something that's not the foam type (or it will act as a pee sponge, which you don't want). I just got back from a trip to Home Depot on my way home, and they carry an assortment of self-adhesive weatherstripping in white. I'd suggest using the white stuff (as pictured by maddog) as it's easy to clean, but maybe pick something flat that will just can be adhered along that flat area mentioned above. It doesn't need to be real thick either, as you're only looking to fill that small gap.
It can be found in the hardware isle (isle 19 at my Home Depot) and cost is around $5.79 for one pack, which is about all you should need.
Hope this helps!!

