I believe if the thief wants your stuff, he's going to get it. The idea is the make it an effort and slow them down, so maybe they will go to someone else's boat. I use torx security screws over a cone washer over a stainless steel washer. The heads of the screws did not quite fit into the cone washers. I put the screw into the chuck of my drill and ran it over a metal file until it fit. Didn't take much. Remember to buy the bit, I already had one. For less than $30 I'm semi-secure and have a bunch left over.
https://www.amazon.com/dp/B01DWDAINA?psc=1&ref=ppx_yo2_dt_b_product_details
https://www.amazon.com/dp/B07QPHNNSS...roduct_details
these were installed over a 1 1/4 inch ss washer. For my Lowrance HDS 9 live, the 20 mm screw worked.