I'm still old school and have been doing hard shell over 'whatever' since I was a kid.
I probably use much smaller paper towels than most to control the surface, my average sized one in N is about an inch square up through 2x3 inches maximum. I precut all the paper towels into various sizes. Normally I'm making my support structure out of cardboard and newspaper and I usually leave it in.
I'll use regular plaster or patching plaster; patching works a lot better if you're going over a surface that tends to leach the water out quicker.
After that dries - but not completely - I'll mix up disposable cups of plaster to the consistency of thin batter and brush it on to add to the thickness and make strength, cover any visible towel joints, may be more than one layer. If you have really low humidity, you'll need to spray it as it sets to really set it up right. That really makes it hard and durable. I have hard shell that's now well over 35 years old and still fine.
I'll paint that with latex earth brown paint and add copious amounts of dirt, ground foam, and blender-ground dried leaves secured with white glue. Unlike joint compound, you won't weaken it with prolonged moisture in this step.
I like the control I have with the surface with plaster. Never been a big fan of foam. With the soaked paper towel layer in there, it's damn tough, repairable, and you have to drill it out to mount trees in it is so hard. By the time I'm done you can't tell what was under there, but there's no visible vertical striation like with foam layers. That's my biggest gripe about foam, I can still see the layer lines in the finished product unless somebody REALLY works hard on the surfacing step.
For modules, it's not light, but the weight is in the surface, not the subsurface, and it seems to take more of a beating over time than foam does.