The first device on offer at Harbor Freight is the Bauer 4 cu. ft. cement mixer.The mixer's product description says it's supposed to be able to handle up to two 80-pound bags of concrete mix in a ...

Buy one bag of the concrete mix. Look on the label for a table that shows you how far it will go. You should be able to make a small thin slab about 2 feet by 2 feet that’s only 2 inches thick.