You will quickly find bigger things to worry about when you start doing shows (e.g., playing the songs "well," keeping a good drummer, actually getting paid, etc.). Otherwise, if this is still a big concern, and you are the least bit handy, I have a few ideas...
You could simply velcro a thin, clear 12"x6" piece of acrylic (~$5.00 online) across the top using the grab handles as mounting locations. If needed you could add a weather seal strip along the back so it seals up the back of the unit to protect liquids from getting in that way. Optionally, instead of velcro, you could use 2 or 4 small clips- find ones that have square adhesive backs and will clip in (correct size) to the grab handles. This might be an even better design. I would look for clips with a slightly larger inner diameter and non scratch material which would allow you to slide the protective cover to easiy access the additional controls as needed. A 3rd option is to use a hing (towards the rear) so you can lft it up to access the controls. There are many very easy solutions to achieve what you are looking to do.
You can experiment with the size of the acrylic but you would want it to be clear and also large enough to cover the vast majority of sensitive areas on top (screen, dials, etc.), while still leaving room for you to access the foot-switches and a little bit of breathing room on the sides (which you could also weather seal, but I wouldn't.) This would be very easy to remove, very inexpensive (<$10) and a practical solution to what you perceive to be a potential problem. I think this would go a lot way to "help" protect the unit from any major spills and even light rain.
