The Little Green Sucker really does suck — it takes air from your compressed air line and creates suction to hold itself to an interior or exterior wall, where it can hold up whatever material you’re fastening. The Little Green Sucker was first made to hold up Hardyplank siding, and I imagine a tool that can hold up a board made with concrete shouldn’t have a problem with other applications, like holding up crown or case molding.
A metal rod with a bent end runs through the tool and serves a couple of purposes: it helps you space your siding equally, or it can hold up your molding. The unit uses less than 1 cubic foot of air per minute at 40 PSI. To release the tool, you just put your finger over the exhaust port.
A set of two will cost you $70, which might be a good deal if it relieves the aggravation of working alone with long material.