You can still do a closed rack system. Just use a piece of hardboard for the back. A large sheet of hardboard from Home Depot is well under $20.00 and it is safe to lay flexwatt on it. When we built our two new melamine racks recently we used hardboard for the back to lessen some of the weight and I made sure and phoned Rich at ReptileBasics to confirm that is was quite safe to lay flex on it.