You want cheap?  Try cutting off the "over the car window sill" portion of the plastic cup holders one can buy at AutoZone and similar for $1.59, drill a couple of holes in the stem, and use 2 small stainless steel bolts/nuts to affix such to the side of the instrument panel of your tractor (which is usually thick plastic - at least it is on Kubotas).  Feeling flush?  Do the same with an aluminum water bottle holder for a bike.  Simple, fast, and holds a 24 oz. beer can  --- excuse me, water bottle --- quite well.