I don't think anyone makes sights exactly for that. Williams may make a universal of the appropriate diameter that you could put on with epoxy. H&R may have some as well. You'll have to measure the barrel and then go looking. Look at both universals and those for muzzleloaders. I'd score the barrel real well and use Devcon Plastic Steel. Would also quadruple check that I was mounting to the centerline of the bore.
If you want it done professionally http://www.guncustomizing.com/
does that sort of thing regularly.
Also, FWIW, Mossberg and Charles Daly have some 20 ga pumps out that are pretty short and only weigh about five pounds. Might be a cheaper route.