Probably gonna have to stack them against the back wall for stability. How many lbs didja get? I'd probbaly try to use the smaller oes on the bottom and wedge the 18" long pc
over them in some way to create a swim through area. Maybe the long pc
slanted from into the bed up to and over the smaller rock on the other side.
Gonna have to play with for a while to get the effect you're looking for.
Also, I like recommending folks push the bottom layer of rock into the bed so that burrowing creatures don't start an avalanche. Also, when I stack mine, I try to imagine me pushing the rock a bit to try to imitate the strongest creature in the tank that might push'm. I know, that may be hard to imagine, but keeping your hands wet, situating and re-situating will ultimately get you somewhere.