You can make a cave with PVC
tube (you pick the size), aquarium silicone and some extra substrate. Use gloves and cover work area. Have a good pile of substrate (works best with matching what you've already got). Smother the outer PVC
with silicone and roll it in substrate. You can leave it whole or cut the bottom 1/3 off the PVC
first to make a U shape... Let it cure for 48 hours then soak in bucket of dechlorinated water for 24 hours and put in your tank!