Mounting a larger 12V battery (20Ah)

Hobie makes a battery mount that clamps to the mast, but it only fits smaller batteries. Mine is admittedly large: 20aH, roughly 7x7x3 (inches). See image below for my current cheesy way of mounting it (I drop it in the hole and use a couple more zipties to hold it to the mast. Looking for something a bit cleaner. Any ideas?

https://imgur.com/a/kvLaIEH

What about going inside... https://www.hobie.com/parts-and-accesso ... ry%5D=thru

Sorry if it wasn't clear, this is inside. When I say I'm attaching it to the mast with zipties, I mean like the same place the smaller batter bracket from Hobie goes:

https://www.paddlerscove.com/12v-battery-holder-hobie/

Or like where this part goes:

https://www.berleypro.com/product/kayak-emb/

I want to secure a much larger battery in that same location using something fancier than zipties and a pool noodle.