https://www.amazon.com/dp/B00ME3ZH7C...p_mob_ap_share
https://www.amazon.com/dp/B07X2TQFKR...p_mob_ap_share
I bought these two, drilled a hole in the case to put the power cable through and epoxied it to make it waterproof. It's nice because when not fishing it doubles as a regular old battery pack (can never have too many). Only ever used up like 10% charge in a full day of fishing.