If you specifically want someone to stand in the hatch I think you will have to take a figure from one of the 1/72 or 1/76 plastic sets and adjust him to fit. That's what Hans did here:
This is the British crew of a British tank touring America.
If you wanted to you could model them as shown, or you could add a holstered pistol and one ammunition pouch, a steel helmet and a gas mask bag on the chest to model them in action.
The leather helmet was only worn by Mk I crews during the Somme in 1916 as looked too much like a German helmet. They also had the leather and chain mail face mask worn throughout the war. Ill link some photos of my living history group doing WW1 tank crew last year, we put a lot of research into the portrayal and was spot on, we're doing it the same at Tankfest this year with the Mk IV tank replica
The article needs a few corrections - the overalls have a top left breast pocket, the only other 'pocket' is an inside one on the right leg for a spanner etc to slot inside (it's a very small opening)