DIY Co-sleeper: Buya regular assembly-required crib and leave one side off. The crib is securely attached to the bed frame with nylon straps. Might be good- but by the time the baby is ready for a crib, I want them in their own room
Try removing the door to your toddler's room and replacing it with a swinging gate. It's better than keep the door open all the time, and the best part it can keep them in the room when you don't want them roaming the house.