To make a compartment, you just need to use something (anything) to have a hollow space in the wall, Then you shape one rock as a plug. It need only be 2 or 3 inches thick and you have to smooth the edges of it that will be in the mortar. To make it stay in place, the plug lips "down" on the lower side. imagine it like a kitchen sink plug that has been warped "down" by gravity. Or like a truncated cone stuck in the wall, with the base out and the point of the cone pointed down at a small angle. And now here is the trick! When you mortar it in place, cover the stone completely in one or 2 layers of that extremely thin plastic stretchy kitchen rap. (I use 2 layers) and tie them like a little bag on the outside. Then when the mortar is set up, just pull out your stone! Actually I usually make them a little bit noticeable so the home owners can find them fairly easily. Because once the wall is in, it can be tricky to find them, even when you know where they are. The one I show is for the shut off for a gas barbecue.