How To Build A Cave From Rocks

Hello I'm not sure if I'm posting this in the right area so feel free to move it to another section if that's where it should be but I am wanting to make some caves out of small bits of rock. I'm looking to build caves as I have panda Cory's and apistogramma cacatuoides and both appreciate caves or somewhere to hide. I'm my tank I did have a PVC pipe cut in half and that was my caves but that was when I had multi coloured gravel and stuff like that but now I have sand and other rocks in there putting black PVC pipes in makes the tank look unnatural but I want to make 2 small caves from small rocks so that the tank still looks natural. After all of that I wanted to ask how do I make a cave and can I just balance rocks on top of each other. I want to use the small rocks on the left and right hand side of the attached picture. Thanks for all your help from Tom

Yes you can just balance rocks on top of each other. Here is my rock formation in the corner of my tank, all balanced on top of each other.
 

 
You have to be careful you don't make them to precarious or they may topple but get it right and you'll be fine. If you're worried you could use an aquarium silicone to hold them in place.
 
I also have a terracotta plant pot in my tank that the Kribs use as a place to lay eggs and that looks fairly in keeping with the tank.
 
Another option would be to use a safe glue or silicone to coat the PVC tube in and then cover it in your sand. Then when you put it in it won't stand out. One of the members on here did this to good effect. I'll see if I can find the thread. I'm certainly considering it if I can find the right size PVC tubing. 

You can still use PVC pipe, just cover in aquarium sealant and then cover in sand, will make the PVC pipe look more natural. 
 
You can use a couple pieces of suitable flat stone and angle these together to make a makeshift cave, can fix these in place using superglue, as long as glue is 100% cyanoacrylate, or aquarium sealant.
 
Do remember to leave sealant to cure 24/48 hours according to instruction before adding to tank.
 
Hope that gives you some idea 
 
Lunar jetman has just got there while was typing this out 

That's what I did for my Peacock Goby breeding pipes.

I've covered pvc with peat moss using silicone. It looked great, but my bn ate the peat moss and I was left with a silicone covered pipe lol

I'm wondering if I could change the level of the sand and have the tunnel angled down and have sand covering it so just the very top of the end of the PVC pipe and I will try the rock balancing cave too from Tom

Before i used the brown inner shell from a coconut as some kind of roofing placed between two flat surfaced rocks then cover it up with smaller rocks and moss. Just pull of the remaining fibers out, sandpaper it til smooth and bring to a boil. My plecos love those

get some d-d aquascape its like a putty thats safe in fish tanks. and will stick them together safe works really good