You talking about "Ring and Bead" open sights? If so, I'lll be honest and tell ya that I ripped off the .50-cal ring-sights from the ProModeler B-24D and B-17G kits, since the diorams for both of those are of aircraft that have had the guns removed (One of a crash-site recovery team getting the gear down on bellied-in Fort, the other is a B-24 Assembly-Ship dio)...
However, all is not lost if you find some styrene or metal tubing, or my personal fav, Q-tip tubing (also the esaiest to slice).. You just "salami-slice" piece of the tube off, then creat the cross-hairs with your own hair.. A dad of paint, lay the hairs across the ring and then, when dry, trim the hair off the outside.. You can also use finely-stretched sprue for the toss-hairs...
OR: Get the molded-on cross-hairs offa the kit-sight, and add the scaled down cross-hairs to it..