I converted my sons crib to a full size bed when he turned 3. But, like you I felt the standard boxspring and mattress was too high. So, I bought what is called a bunkie board instead of a box spring. It is about an inch thick but very sturdy! My husband or I have slept in the bed with him sometimes and no problem. We bought it at USA Baby for about 100.00 I think. Now I don't worry about him falling so far to the floor (If he does roll out- now he is 4 1/2 and is too big for rails :) ) and it is just his size to get in and out of. Many of my neighbors have done the same thing. (It just like what goes on bunk beds) Not many places have this and when I checked around USA Baby had the best and sturdiest. Good luck!