Sigma has discounted their telephoto zooms, but... with many solar filters now out of stock, a telephoto zoom without a solar filter (or some type of solar protection) will do you no good for the upcoming eclipse.
However, all is not lost. Amazon
still has some Thousand Oaks Solar Filter Sheets
that you can use to fashion your own lens filter.
If you already own a square filter holder, you can sandwich a square piece of film between two piece of stiff cardboard with a holes cut out that match the size of the end of your lens. Otherwise, if you don't own a square filter holder, you can make a cardboard filter holder that slides onto the end of your lens that is held in place by gaffers tape or a rubber band.
If you go the DIY approach, safety first! Be sure to guard against light leaks and ensure that your filter will stay in place throughout the event.