Need to add something to increase height and sq2 of keyboard tray

I read here my mouse height needs to be slightly above my elbow/arm.

  1. Optimal Mouse position - sit back in your chair, relax your arms then lift your mousing hand up, pivoting at the elbow, until your hand is just above elbow level. Your mouse should be positioned somewhere around this point.

So I was thinking a large slab of something very light and plastic-y might work which I could cut to fit on top of my current mouse/keyboard tray. I don’t know where to get something like this, so when my brain thinks of what fits, the first thing I think of is cutting boards. But they may be a bit too heavy I think and not wide enough.

Any suggestions?