For Rapid Fire, there is a scenario in which the Germans have to cross the river Bug (aptly named "Crossing the Bug" with Tauchpanzers etc) and the Russian defenders get a emplaced T-26 turret.
Much googling later I have this:
....which isn't much to go on. So I decided to keep it simple. 40x40mm base, 2 layers of plasticcard to raise it, brass barrel, drill a hole for the turret ( Pegasus BT-7 kit) so the turret can turn and a quick paintjob with some tufts. Nothing grandiose but it gets the job done.