All parts were purchased at the local Ace Hardware store.
I used a trash can (32 gal) for easy cleaning and construction. A 55 gal blue barrel could be adapted for increased water needs.
1 trash can with lid
2 plastic downspout extenders (accordion style), fitted to the size of your downspout
1 bulkhead fitting (this may be difficult to find at HD or Lowe's, but search Ace or online)
1 boiler drain valve or your choice of hose connection
1 tube silver gutter sealant (much better than silicone for this job)
At first I tried silicone to seal the seams, but it wouldn't stick the the trash can material. My next move was to try Seamer Mate, found in the roofing/gutter section of home depot. A full-size caulk tube is not necessary, and you have to throw it away after you open it, so try to find the smaller tube to save some money.
Since using Seamer Mate, I found the Lowe's version. I can't remember the name, but it's found with the gutters and it comes in a caulk tube. It's clear and you don't have to throw it away after opening. Much more economical. This stuff has a million waterproofing uses, and what great adhesion!
During winter, the barrel can be removed. Just take the top accordion and reconnect the two sections of downspout for flawless performance.
If the trash can is too much of an eyesore, you could always build a little house like I did here. I found some old doors and used them as my walls, with just some planks as a roof. It doesn't have to keep out water, just hide the barrel from view.
I put hinges on the front for easy access,