Ya, hydrogen is lighter than air, so it wants to rise. You aren't making that much either.
My thought for such small battery space is the guy saying 3" to the roof is plain nuts.
First, hydrogen wants to rise. Second during winter the gases from the batteries in storage like that should be warmer than ambient air, warm/hot air wants to rise.
Any ambient air in winter is probably colder than the battery space, so it wants to drop.
Depending on the discharge and rate of charging you may not be making enough hydrogen to even worry about really, any venting should be fine.
There should not be any sparks, flames, etc in the battery area.
If it were me I would run maybe 3/4" pipe in and 1/2" out, keep it away from windows near a fire source like the wood burner, and run it about 2' above the box/ground.
In summer you lose ambient cold air warm air flow, but for just a couple batteries it should not matter, hydrogen still is lighter than air and wants to rise.
Do you run a 3" pipe 20' in the air to vent your car battery?? Of course not, and it probably produces far more hydrogen than your storage batteries! And with a faulty wire it's right next to igniting sparks also! On this point I shall say today was a rough day for me. My truck has a starter flywheel problem, worse when hot and engine stiff. So I got stuck for 2 hours today, crank, sit, crank sit, ran battery low and was jumping from a booster pack. So it finally started with maybe an 11V battery, I found a bad wire sparking, 100 amp alternator charging high, never blew it up! I did of course replace the wire when I got home. Point is, that battery was probably making more hydrogen at heavy charge for an hour than 4 of my L16P trojans in normal use, and it had a spark close by.
BE SAFE. Don't be paranoid though.