Find out what they are made of. Likely ABS. Find out by putting a drop of ABS solvent-cement in a corner and see if it disolves the plastic.
Then, get a piece of scrap ABS drain pipe from a plumber. Shave some bits into a small pile. Using a wide flat tip, melt them into a glob using your soldering gun. Don't burn it. Then glob it into the hole and melt/weld it in with your soldering gun.
If not ABS, maybe HDPE but not likely. Find a scrap of watermain pipe = HDPE
You can do the same thing chemically by making a black paste with ABS shavings and ABS cement, but you don't have a lot of working time with that.
