Do you have any of the Rubicon figures from their Half Track add on kits , They should be slim enough to fit if you place their chest toward the gun breech , just adjust the head to be sort of looking forward . You could also just build up a rectanguler shaped box , round both sides of one end ( for shoulders ) and add a spare head , from a top view it will look like a crew man . I managed to get a commander in the BA T-34/85 hatch . He's just a couple of short pieces of plastic strip , the top one shorter than the bottom one , giving the idea of a shoulder , than added an arm and head
In your turret the loader would be as far to the rear as possible , that L71 gun had long ammo , you could notch a plastic upper torso so it fits partly over the interior turret race . This thing would have been used as a long range sniper , big hard hitting gun , very light armor a lot like a supersized Hellcat , doing the old shoot and scoot probably the commander would have been out of the turret looking for victims ,and places to move the vehicle to set up the next ambush . Have no idea why the pictures show the gun with that high elevation , its an anti tank gun not a howitzer . Of coarse they could have planned to mount both guns in the vehicle model like the Nashorn / Hummel