I have owned a few sets of cheap lights.
I currently have a $40 set of slim fog lights from Kragen Auto under my rear bumper. They are fine for backing up in dark areas, but I would not want to use them to see past a 20 foot boat and trailer.
I have also owned standard "PAR 36" 35W sealed beam lights commonly available in Harbor Freight
, NAPA, and truck dealerships. They come in a rubber housing and are about 4 inches in diameter. The ones I had were "tractor beam" pattern. They were a little bit too diffused to be useful past 20 feet IMHO. There are many different sealed beams that will fit into a PAR 36 housing, including 100W aircraft landing lights.
Never did try those, but I am told they throw light all over the place.
What I would recommend is something like this: http://shop.advanceautoparts.com/web...zoneAssigned=1
I had a set of those as backup lights on a past truck. They would satisfy the 20 foot requirement. They are referred to as a driving light pattern, but there were terrible at any kind of speed, didn't project far enough out to be of any more use than my high beams. But at low speeds they were fine, that's why they got moved to the rear bumper.
Try to get a plastic housing. Anything metal in the "inexpensive" range is going to have cheap paint and chrome on it, and will rust fast.