I wanted a simple design that I could cast easily in concrete. 
this is what I came  up with. I might have to modify slightly to increase the thickness of the bottom. We'll see. I'll cast another one and test the strength and get back to you :D (also get some pics)

I printed the mold in ABS so I Could smooth it more readily with Acetone. 
this worked well enough… don't let it sit long…

Then I just oiled it a little before pouring the concrete in. 

The ABS has enough flex that the ashtray pops right out.

*** Edit ***

I've taken the time to make it a lil nicer, used some JB-Weld to attach some cork to the bottom. I think it's turned out very nice. So I've added pics!

Enjoy!