Don't let someone tell you how to organize your shadows... You are the ARTIIIIST, after all! To fit more fabulous shadows onto my Pro x15 palette, I wedged a small flat head screwdriver through one of the empty shadow spaces, and underneath the top plastic piece. I pried it up just enough so I could grip it with my hand...
Be careful here, if you give it too much muscle, it will snap and could fly across the room- possibly injuring an innocent bystander (or you!). Slowly pull one end up, and if you get it just right, that piece will pull right off.
This can work for many brands of palettes, existing Mac Pro x15's, and mac palettes that are smaller as well... so play around and see. Mac is making a new 'blank-canvas' large palette that you can customize now, I purchased this newer generation palette with an optional plastic liner for lips. Sounds like we won't be breaking the inserts out of mac pro palettes for long! Good work, MAC.
Thinking outside the box, errr, palette: you can get nuts with products, too: invite your foundation powder to the party, blushes, bronzers, cream blushes, whatever!