My new and possibly forever favorite now is Type S Mortar-- it's cheap, and you can use literally anything as a form/base (there's a guy on YouTube that makes super-realistic rock by putting Type S over crumpled up cardboard boxes.) Added bonus, it grips like nobody's business. Makes Amazon knock-offs feel like they're exotic compound.
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I have gotten away from using scrap to make anything for crawler courses because it looks like a pile of scrap. That's a great looking natural course.