Trap stocks for 870s are easy to find. Remington carries them, so do all the aftermarket vendors like Wenig's, Boyd, etc. But....
I see lots of good work done with standard stocks on 870s, 12s, and so on. The trick is to have a shotgun that prints a nice pattern a little above Point Of Aim. You want about 60% of the pellets hitting above POA, or more, and 40% or less below.
Of course, stock fit's crucial.
Actually, I like my field guns to hit like this also. We shoot more flying stuff that's rising than falling, and it builds in a bit of vertical lead.
Pattern your shotgun as it is, and see where it's hitting. If you want a bit more elevation for that 60-40 pattern, the stock can be shimmed or a glue on pad like the Cheek Eez can be added. A rule of thumb for stock fit is that a 1/8" change in the stock will move the pattern 4 inches at 40 yards. Since we usually break our birds at 35 yards or so for 16 yard singles in trap, this can help us "Dial it in".
As for long bbls, the advantage has more to do with balance and swing than longer sight radius. Remember, we're looking at the target, not the sights, when we shoot flying. That 28" bbl will work well, if it prints like it ought to. A slightly more muzzle heavy shotgun is easier to keep moving.
My little overpublicized parts 870 has a 21" bbl, and I score a bird or two lower each round with it than I do my 30" bbled 870 TB claycruncher. I have to work a bit to keep it moving.
HTH, and ask any more quesitons you have. The only dumb questions are those nobody asks...