Creating SHB oil/soapy water trap in baseboard?

I have a original 8 fr flow and I would like to modify my bottom board. I have switched my hive to a freemam bottom board. It has helped my SHB issue. I have pulled my tray out of my bottom board as I was going to see if I could fit any tray for oil/soap water and bees were there. Has anyone done this? Suggestions? Thought of closing the back to keep bees from going in?

The first thing to do is to prevent the bees getting into the tray area. I have made up trays that contain cooking oil so that whatever falls into it die and have not just escaped from the bees chasing them to regroup and escape. SHB is an issue with me and it’s nice to see so many dead SHB in the tray when I remove them to clean after 2 weeks during my inspections.

I was going to get a tray to put under, in the slot there. Worried that bees would go in. I use ones with soapy water and it does kill the SHB. But I bought them premade for my new boxes. I thought I could just slip the tray into bottom my flow hive, Did not know if I should close the back?