I made mine from a 24" regular floor squeegee. The one I have has an L shaped metal frame for the rubber blade. I drilled a hole thru the metal frame at each end to accept a 4" long stove bolt with a couple of locking nuts for setting the screed height.
It works ok as long as your not trying to go over heat wires.
Lowes actually sells an SLC rake from Henry. It has adjustable cam style screed guides. Pretty inexspensive too.
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Randall