I was looking at a new sprayer for the three point hitch. I asked the guy about a foam maker. He said make your own - Get a 14-16 gallon spray tank, get a 12 V tire inflator with good air flow, put a one inch diameter tube into top of sprayer tank, and the hose of the inflator into the bottom under water in which you put in a good quality dish soap and you have yourself a marker. He said it won't put out the foam at six foot intervals but proably irregular intervals of about 20 ft. That works for me. Haven't done it yet, but have the air compressor, need time to put it together.
Billy