Just get any brand of sealed battery and use a flexible 10 watt solar panel to charge it with, get a proper PWM solar charge controller. Jaycar sells a sealed one that will do the job.
https://www.jaycar.com.au/12v-8a-wat...oller/p/MP3720
Put the flexible solar panel on your dash, run the wire through the firewall, place the charge controller in the engine bay.
DON'T get a 5 watt solar panel. Its too weak. Something like this is good:
https://www.ebay.com.au/itm/10W-12V-...G/203064026448
43cm x 19cm is perfect for up on the dash.
DON'T move it around too much when its up on the dash they can be fragile.
This is what I did before I bought a LifePo4 battery and waved goodbye to lead acid. May have cost me $500 but it was worth every penny, would've replaced the battery twice by now.
Minimum of 2,000 cycles, 8,000 cycles more likely.
Doesn't matter what the charge state is, can be half flat and you don't need to worry about charging it straight away.
Has more than 1,100 cold cranking amps no matter what the voltage is, weather it has 10% left or 100%.
Design life of 10 years. Is about 3-4 years old and still going strong.
No need to worry about under bonnet temps as they work better at 60c than they do when they are colder...
If you're smart you can gut out an old lead acid battery and make up your own out of 38120 cells but I wasn't in the mood.
Brand is "powerstart" that no longer exists on eBay.
To make your own you just need to find a case, or buy a case, for this:
https://www.ebay.com.au/itm/Headway-...K/293128877075