If you find a sturdy one with a lid that cn be clamped on, you could half-fill it with soil mix, clamp the lid, and then mix the soil by rolling it around.
Or set it on top of two skateboards or roller skates, and turn it by hand like a cement mixer.
But it mgiht be easier too mix soil in a wheelbarrow or on a tarp, with shovel, hoe and rake.
If you do it on a tarp, you can mix soil fom the edges and bottom back into the rest by pulling the tarp up from first one side and then the other, rocking the mass of soil back and forth.
55 gallons = 7.35 cubic feet (around 440-450 pounds of water).
I think that would provide one inch of water to 88 square feet:
a bed 22' x 4'.
