Make Crib Out of Bottom Bunk

My husband and I just bought a Shadow Cruiser 282BHS. It has 2 corner bunks in the back. We are looking for some type of bed rail for the bottom bunk for our 18 month old. We can't really use a traditional bed rail that slips under the mattress because the "mattresses" are only 2 inch thick foam pads.

Any suggestions would be much appreciated!!