Keeping My Rolling Backpack Dry?

I take a rolling backpack when I go to school (it's a backpack on wheels that looks like the rolling luggage you see in airports). It's raining cats and dogs out there and I am wondering what I can rig up to keep the backpack dry. The heavy text was over $100, the calculator ditto and I want neither to even become damp.

Although I have another backpack on wheels that's a cheapie and wouldn't mind if it becomes wet, I need to keep the contents safe from rain and puddles, many of which I'll be wheeling through. Any suggestions?

I am currently thinking a giant ziploc bag in the interior, but when I put the book back in, I just know that there will be water in it (it's raining that hard). I just wish it would rain over the lake where we get our water supply. The darn thing is something like ten feet down and lowering.

Thanks,
Holly from Richardson, TX