While I was messing with my Aston Martin's Ford/Volvo alarm module battery (a sealed unit with an NiMH battery that goes bad), I suddenly remembered that NiMH batteries are lighter and more durable than lead acid and very cheap too. They generally aren't meant for high current but that's where this thread's focus comes in
On Amazon, you can get 20 Tenergy D-cells for 97 before tax (16 pack + 4 pack), and then configure them 10S 2P for 12V and 20Ah. This is a little bit less money than buying 4 unknown brand 20Ah lithium iron phosphate batteries (which have limitations on cold weather usage). Alternatively, you can buy no-name batteries for half that price on Aliexpress, or used batteries out of Prii.
A little bit more expensive than lead acid, but it should last longer.