If you have nothing in the way (shifter triggers for example), you can use cheap "thumbies" (1980s style gear shifters, $5) for a thumb throttle.
These work great for shifters, and you can put one under the bar, loctite the screw, and leave it just a tad loose, so the lever is pulled back by the carb spring. It is important to use thread glue on the screw so it doesn't loosen and come out (the shifter will come apart). When used conventionally the screw is tight, so not a problem.
If you get the pair from Niagara Cycle Works for $10, it comes with cables and housings. They also sell the single shifter for $5, but I don't know if it has cables or housing. Probably at least a cable.