[vibrolax]

If you want a near turn-key solution, there is this: http://www.mini-box.com/DCDC-USB?sc=8&category=1264

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

[vkuruthers]

Thanks @vibrolax, that does look ideal. I was going to muck around and design the circuit myself (hopefully based off a solid reference design), but the COTS setup you listed above looks just like what I need, so I think I'll go ahead and get it once funds allow :) Thanks again for the tip.

[vibrolax]

You're welcome. Another possible solution for you is a dc-dc laptop brick like these: https://www.powerstream.com/ADC-p006.htm.

Fit your gel cell with an auto accessory socket, then plug in the power supply.