You just need a reducer of some sort on either side of your bulkhead fittings to take your 1 1/2" pipe down to 1" or whatever size you want. If you don't want to glue the reducer in, you can use a bit of teflon tape around the pipe to seal it.
Here's a crazy idea...Could you maybe still make the rock in a sandbox, but use use twisted/rolled/scrunched up wax paper to form the hollows in the rock?? Just make sure you leave some paper hanging out the bottom so you can pull it out when the mix hardens.
This way it would be more...
