I want to make mine RobC style. Covered with Marine Seal and substrate

Thats the way I did as well...like I said, just make sure the T cant get hold of the styrofoam. If it can get a good hold, it will pull it apart.

Styrofoam and polystyrene is almost the same thing.
One is used for packaging and the other for building and sculpting

Try builder warehouse, Mika, any hardware store for that matter.

Also watch out for crickets they will destroy any design and all designs, they have a nasty habit of chewing through the paint and chewing holes into the polystyrene.
and then as Bonzai has stated some of your spiders will also break pieces of polystyrene off.

But that is just part and parcel of keeping tarantulas with designs!

I've been wanting to try and use high density foam in stead of styrofoam. If you use the same shaping method as RobC I think if should work, and be more durable than styrofoam. I have also noticed that most trimmers have a lot of off cuts that they just threw away.

Do you get high density foam at the Builder's places as well? And what are trimmers, where would we find those?

Look for un upholsterer in your area. They will have high density foam or send you in the right direction to find it.