Smelly drain needs a valve

So, there's a foul smell coming from this drain in a commercial space I am the new tenant of. I have seen various one-way valve/flap solutions for this online but am unsure what product and size to purchase due to getting burned by nominal sizes not matching actual sizes in the past. I am looking for a link to a thing I can purchase that would solve this smelly problem. To my knowledge there is really only an odor problem and not an issue with liquids flowing the wrong way.

Thank you Reddit. Ruler for scale! https://imgur.com/a/so4me

Update: So it seems like the trap was dry as per all of your comments suggested. Good ideas all around! I also purchased one of these as added insurance. It seemed to fit well in the smaller diameter pipe the drain connects to: https://www.amazon.com/gp/product/B00W2FPBJG