I went redneck on this one and have two 32 gallon plastic trash cans as my rain barrel.

For the last three seasons I was just dipping the watering can into the rain pig and it worked well.

This year I coupled two cans together using two electrical-box bulkheads designed for watertight coupling to flexible conduit, a length of hose, and some hose clamps. I did this about 100 mm above the bottom so that as one barrel fills the second seeks the same level.

If you sneak a T fitting into the hose between the barrels you can have a hose to fill watering cans with using the natural pressure.

The trash cans were about $10 each on sale, the bulkheads were about $1.75 each, and I had the hose clamps around the shop.