I reviwed a few web sites for a good first-flow device but, as you noted, a lot of the sites don't even mention them and I got distracted before I found something I thought suitable. Sorry.

I note that the pdf link Stevenpd provided features a picture of a workable version on the first page. The 'T' at the top directs the first few gallons to a dead-end. Once that initial eight feet of dead-end pipe are full any further flow goes to the tank.

Some first-flow setups use a series of reducers to keep the first dirty water stays in the tank but the setup pictured would IMO do a pretty good job because the vertical length of pipe used as a first-flow tank is narrow enough to limit mixing.

The pdf brochure is marketing something they call "first-flush" which is a valve the control draining of what I call the first-flow tank. Different language but same idea. The first few gallons off the roof gets diverted and discarded so bird droppings and dust, anything that might dissolve in rainwater, get excluded, for the most part. Larger trash, bugs, and stuff that might float out of the first-flow tank get removed by a simple mesh filter. There is a nice filter in the pdf but I've seen a piece of fiberglass window screen inserted at an angle in a pipe.

Of course, obviously, the good people at RainHarvesting.com have engineered solutions that they will sell you. The people I've been around were more shade tree mechanics and they designed and manufactured all their stuff from plumbing fittings they buy at the hardware store.

Using a simple plumbing 'T' as a first-flow diverter, and letting its filling trigger flow to the storage tank is good shade tree engineering in my book.