Motivated myself to build my air dam today. Got a 2x2x8 and some fat screws and washers at the hardware store, and coupled them with some 5" lawn edging I got for free. Total expense: $8 and about 2.5 hours of work.
The air dam comes off with four screws in a few minutes. And it survived some mixed driving this evening. It scratches on curbs, but so far the lawn edging seems to take it.