I don't have a Lopi Freedom but I was looking into battery packs for my FPX stove's blower for the same reason you stated. I ended up getting Tesla Powerwalls and solar for my home so it is now a moot point. However, if I was going to go with a battery pack, I would have needed to change my current hardwired blower setup to a 3 prong cord for a regular outlet.
With a 3 prong plug and cord running from your blower, determining which battery pack you want is just a matter of math. I'm guessing that your Lopi blower draws about 50 watts when it's running. You can likely get a precise # from your Lopi owners manual/spec sheet.... although you may need to convert amps to watts... watts = amps * 110 (volts). That means it will take roughly 50 watt hours of battery capacity to run it for one hour. If you want to be able to run it in a grid down scenario for 24 hours you would need a 1200 watt hour battery.
Jackery is one of the most popular brands for portable battery power packs but there are dozens of others. They are typically sized in roughly 1000 watt hour increments. A Jackery 1000 (
https://www.jackery.com/products/jackery-explorer-1000-v2 ) has 1070 watt hour capacity..... so doing the math, that would be good for about 20 hours of blower run time. Personally, I wouldn't want anything smaller than that in your situation.