I made them with straight portland and sand.
Wash the sand befor use.2 part sand 1part portland cement is the mix
find a box about the size you want to form slightly bigger. Put your damp sand in the box and form where you want the open cavities to be. Then add your mix "don't mix it to wet. Mix it were you can mold it in your hand."spred it out over every thing that you want covered about 1-2inches. Leave a open spot were you want the cave openings.
Let it set for24hr. slowly dig out much sand as you cane than lift it out slowly it is still green and can brake easy. Then rinse of real good.
Put it in water and let it set. change the water every day. Do this for a week.
pull the mold out and rub all the extra cement and any sharp sopts. Spay it down with water and spray the tub holding the mold.
At this piont you should start testing the water the mold is curing in. Use tank water when testing. when you like the numbers let it set for a few more days and the numbers are still good your mold is cured. When you put it in your tank test the water twice a day for about 2-3 days. If the
pH climes to high pull it out and let it set in a curing tank longer. But with good testing before hand you should be alright