You can also purchase a 12 volt battery about the size of a brick. Vexilar makes one as do several other companies. These are rechargeable, weigh little more than 2 6 volt batteries, and will power pretty much any standard fishfinder. That will open Your options.
as said , there are 12v batteries that are downsized.
when I first did the install about 15 years ago I used a home security system back up battery.i had to special order it through a motor cycle shop it was an interstate iirc
it was 12v ,18amp
I could run the fish finder/chart recorder for about 3 days off a charge.
it was small enough that it fit in the side pocket of my soft sided tackle box.maybe 8"L X3"W X5"H
You could use one of those jumpstart devices, much lighter than a full size battery,and mine has a cigarette lighter receptacle and a USB port. They come in different size's and configuration. Check the auto parts store,TCS, Cabelas, Bass-Pro, etc.
Try one of these. ww.batterysharks.com/Universal-Power-UB1280-D5743-p/UB1280_B12-7.htm?gclid=CjwKEAjwh6SsBRCYrKHF7J3NjicSJACUxAh7pHhK2mH
I use mine for ice fishing and charge it at the beginning of the season and it lasts until the end of the season. Around 80 days a year. Weighs around 2# Measures around 8 x 4 inches. Have had mine 9 years now.