Hi & welcome.
I've never seen/heard of anything ... that recess is pretty shallow so something in it would likely come out if you went over a bump. I have the MB accessories booklet for the W639 Vito and there's nothing in that. The 2+1 seating is called "Comfort rear passenger 3-seat bench (1st row)" (factory option US4) ... might help you search.
From a purely practical POV something generic like this from Amazon etc. (held by the seatbelt) would do the job, even if it's not the most stylish thing you've ever seen
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