Ideas on how to raise crib mattress

Alyssa

I have the crib up against my bed with one of the sides taken off (its a convertible crib-to-toddler bed) and i have the mattress raised all the way up, but it's still about 4 inches lower than my matress. Im definitely not adept enough to do anything to the crib itself, but does anyone have ideas on something cheap i can maybe put under the mattress to raise it?