Why bother? Most watering cans I found on-line were metal with prices ranging from $25 to $50 each. I made my sprinkler head for about $3.50.
Step 1: Parts needed
Step 2: Fitting the male adapter
The factory threads on the watering can spout were 3/4 inch in diameter. The male adapter slid over them with a close fit. I drilled a hole about 3/16 inch in diameter through the sides of the male adapter. Hot glue fills these holes to lock the male adapter on the threads. I coated the threads on the water can with hot glue and quickly slipped the male adapter over the threads. I also pressed some hot glue into the two holes I drilled. The hot glue makes a good seal and very firmly secures the male adapter on the watering can spout.