In my opinion, an over-the-side arm is less ideal than mounting to the Lowrance Ready cavity on your boat. The arms cause tracking issues and can get in the way of your fishing. Options for fix mounting to the bottom of your kayak are by using either the Hobie kit or the BerleyPro kit. Care must be taken, but the transducers can withstand some scrapes and abrasion.
Here's the Hobie part you'll need:
72020207
PLATE KIT, LOWRANCE READY TOTA
US$ 39.99
If you're interested in the BerleyPro version, check out their website:
https://www.berleypro.com/product-category/hobie/