I have noticed that when there is food it usually keeps them quiet but i hadn't thought about opening the door when they are quiet so thank you.

So you have taught them that they need to bark/work for at least 5-10 minutes before it will work.....

So try throwing 2-3 pieces of kibble in different places at opposite ends of the room during a stay, so they can't snaffle it up before you have even shut the door.

Verbally release the dog to find it (rather than bark) as you shut the door, open door WHILE STILL QUIET, repeat and SLOWLY and gradually build up and reward increasingly long quiet time.

You can also teach a "quiet" command. Use this when they bark (firstly when the door is open) and once they understand it, use it when barking behind the closed door and again only open the door as a reward for quiet.

Initially open the door as soon as they are quiet, or mark that point as you open the door, and reward with an opened door.

Again gradually build up the quiet time required to earn teh opened door reward.