Shouldn't make any difference as far as I know how thin or thick the plaster is. Big problem is if it's really thin it's going to run everywhere unless you carefully dam it.

Peruse the directions on the container.  Some of the premixed stuff advises not thinning it.  Keeping in mind that if water is the thinning agent, thin mixes tend to crack as they dry.

I actually had been researching this lately and came across a great article by a sculptor that went into great detail re: the properties of plaster. I wish I could find it again. If I remember correctly, varying the water/plaster ratio outside the "window" of the ideal mix is not recommended because it messes with the plaster's ability to cure to its strongest in the least amount of time.

  If it were me, I would do a test pour around a similar object as in your diorama. Experiment for safety's sake.  I think if you vibrate or shake the base after the pour, you could get the plaster to settle in nicely around your object.

Chris

I've not tried it but I've heard of people that mix in some Elmers type glue with the plaster and water. I take it that this gives some flexibility to the end product so it doesn't crack as easy.

Cell-u-clay (do they still make that anymore?) was able to be thinned down pretty thin.

Yeah they still make Celluclay but it will also crack and worse, shrink if too wet. I think I'd use plaster, mix it as thick as possible, and then vibrate the base 'till it settles where you want it. This advice presupposes a single level is desired. Regards, Mark