Yay!! I get to be helpful for a change!
I have the same problem... I have parties and it just takes one person to put their drink on top of a game and they were all doing it!
I actually brainstormed after seeing the 1up arcade tour video, but I believe they just had a board fastened to the side of a cabinet.
Like you, I didnt want to be screwing into the sides of my cabinets.
Here was my soloution!
(Materials and Measurements at the bottom)
Untreated 1x6x6 Fence Board (sanded till its pretty and the corners are nice and round.
2x4x8 cut to make the frame.
Wood Screws (for the frame)
fancy bubble top screws to fasten the fence board to the frame
Measurements:
all 5 2x4 lengths are 30"
Top Fence board is 32" the front corners are extra rounded (sander)
front facia fence board is 33"
Tools
Chop saw
Orbital sander
Drill (I predrilled the holes so as not to split the wood)
My cabinets pictured is a Galaga and a Dynamo (with badlands kit) so if you have any non standardish cabinets you might want to measure for the ideal height and depth so they donty stick out past the kickplate.
I may paint them in the future... but for now they work PERFECTLY!