ABS forms well "by hand" using a shrinkwrap gun or other air heating gun (gotta be a lot hotter than a blow dryer), if you are just pushing it down (using welding gloves or similar) to a surface or shape.
You can also make your own vacuform molds out of wood, drill them full of lots of little vertical holes (about one every 1/2" is good, plus more around the edges or details). Easiest to make a "positive" mold, where you are just going to pull the plastic sheet over a protruding shape, rather than trying to get it down into a negative mold properly. If making a positive mold, just make sure you have a full rectangular shape for the thing that's smaller than the sheet of plastic you're going to use, with the flat "unused" area of the rectangle also drilled down at least around the edges of the molded area.
Bolt a 1/2" deep cookie sheet to the bottom of the mold, using foam, weatherstripping, rubber, cardboard, or your choice of gasket material to try to make sure as much air comes thru the drilled holes in the mold as possible. Hole-saw a hole large enough for your shop vac's or upright vac's stair/furniture hose to *tightly* fit into, and seal that if necessary, too.
Make a frame (metal is best) to clamp the sheet of plastic to, that is small enough to fit in your oven. Those flat-faced extruded-aluminum picture frames, a pair of identical ones, work well enough, with vise-grips or c-clamps trapping them on each side of the plastic sheet.
Take the whole thing to your preheated oven (check the temperature guides from plastic companies for whichever type and thickness you are using), and let it get hot enough to begin sagging. While you're waiting, which wont' be long, turn on the heat gun and begin preheating your mold surface. This works really well with plaster molds, but if you have any detail in them or they have thin edge areas, plaster molds don't last more than a couple of uses at most.
As soon as the sagging starts, then turn on the vacuum, and open the oven, and pull the hot plastic sheet in it's frame down over the mold, so gets sucked onto the surface. If it doesn't look like it's conforming, hit those areas with the heat gun fast and close, with the vacuum still running.
A better option than a vacuum if you have good seals on everything and you've got a hinged frame and built-in heaters instead of using an oven, is to reverse-attach an air compressor to it's tank, and suck all the air out of it. Then hook that to the vacuum plate on the bottom, and flip the valve open as soon as the hot plastic is seated on the mold.