We used this one initially on DD1s bed: http://www.amazon.com/Regalo-Hide-Aw...words=bed+rail
I think it worked fine on a regular/standard twin mattress with box spring.
We use a pool noodle now because even at 5yo she still has issues...she likes to hang a leg over the edge of the bed and one wrong move and she's onto the floor. Honestly, the pool noodle works great and had I known just how great I wouldn't have gotten the bed rail.
Other things we did when DD1 transitioned to a big bed was start with just the mattress on the floor. Then after a while we put it on the box spring. Then after a while we put it all on the bed frame.
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.