I have to agree with Pig, if he always lives in the same place, just get some pvc (thats a similar size to his "cave"), cut it at an angle (would be ideal, but straight could work to) then shove it under the rock. Or you could cut the PVC down the middle so you end up with an arch, then cut the arch at an angle, so you end up with the back of the arch at or near the bottom of the tank, and the top up and open, creating a shallow wedge shaped cave.