Mini-box has a number of dc-dc converters and battery charging systems for gel-cell and other batteries. I do not have personal experience with their products, but I'd try them first if I wanted something other than a hack.
For hacks there are tons of boost converter boards on eBay/aliexpress, but these typically don't have chargers, short protection, or other features you might want if you need more robustness
Mini-box has a number of dc-dc converters and battery charging systems for gel-cell and other batteries. I do not have personal experience with their products, but I'd try them first if I wanted something other than a hack.
For hacks there are tons of boost converter boards on eBay/aliexpress, but these typically don't have chargers, short protection, or other features you might want if you need more robustness