I've done similar with tin foil. Buy some cheap foil that you don't mind wasting and pull out some really long sheets. Long enough that you can fold it up to be enough to overlap around the pipe, cover several inches on either side of the union, and be at least 1/4 inch thick or so. Make two of those, and wrap the first really tight around the union, then the second so they overlap in different places. Crank down some worm clamps over the top and you should be good. I've used that for several weeks before.
I've done that several times. The first time I didn't make it thick enough and it tore, but I haven't had any issues with thick layers. It would also help to smoth down any sharp edges.