There were six basic types of German railway anti-aircraft wagons:
Geschützwagen I(e) - 20mm solo or quadruple
Geschützwagen II(e) - 37mm solo or 20mm quadruple
Geschützwagen III(Eisb) - 88mm
Geschützwagen IV(Eisb) - 128mm
Geschützwagen(e) MG151/20 with two turrets - 2x 20mm triplet, solo or quadruple
Geschützwagen(e) MG151/20 with one turret - 20mm triplet, solo or quadruple
See Waffen Revue Nr. 24, pages 3883-3911.
As far I know, the Geschützwagen I(e) type was built on French and German flat-wagons. Years ago when I built one I used front and rear part as well as wheels from the Planet Models Gattungsbezirk Linz offeringhttp://www.cmkkits.com/en/afv/vagon-gattungzbesirk-linz/
) and the rest was a scratch-build from Evergreen profiles:
Two-turreted Geschützwagen(e) MG151/20 offers KORA Models as a complete resin kit under label "Flakwaggon Drilling":
Scale drawings of the two-turreted Geschützwagen(e) MG151/20 are in the Paul Malmassari's Les Trains Blindes 1826-1989. Editions Heimdal, 1989.
Pictures of both single- and two-turreted Geschützwagen(e) MG151/20 are here:
Scale drawings of all mentioned flak-wagons are in the Czech railway enthusiasts journal "Svet zeleznice" Nos. 34, 35 and 36.