You can make a clear vinyl splash guard for any bag.
You may want to use a waterproofing agent on your bag to help repel water. Though if you have any scotch guard, especially older stuff, you may want to throw it away. The active ingredient is evil, toxic stuff that has been banned by the Kyoto protocol.
I have wanted to cover the bottom of my bag with a hardcore rubber compound. I was thinking of something like tool dip, or a silicone. Something where it could sit in a puddle for hours and be just fine.
Finally.... if a bag gets damp you must remove your camera and recording gear from it. There is a saying among serious photographers that bags are for transporting gear, not storing it (lenses get destroyed by internal fungus that etches the glass). Even if the bag is waterproof, the risk is that once damp, the inside will become supersaturated with water vapor.
I once heard a jam cruise story about schoeps being stored in a garbage bag. So, ya know... Consider any solution