Help: need to extend monitor stand

I'm hoping someone can help me out with this situation. I've out grown my computer desk, but do not have the money yet to buy a new desk (found one for $300 range).

I want to be able to use my three monitors because I work from home and it does make my job easier. Although, as you can see it's really pushing the limits and I was hoping to find a way to attach maybe 6-8" of plywood or some type of extension on to the top part of the monitor stand, there is about a 3" lip on each side of the monitor stand.

I'm afraid someone will bump my desk and since the end monitors are sitting only on 3 corners each they will come crashing down.

If you have any ideas, that would be great. I've thought about the idea of clamps but I'm afraid that the plywood might be too heavy and crack the MDF (I think it's MDF anyways).

I've attached a photo here to give you a better idea of the setup: http://i.imgur.com/OLQWi4G.png