pressure wash, hand scrub, then build a fire pit and a grate to put a large metal trash bin on top of, fill 'er with water, wait till she's boiling, tossin the wood, wait a day or two, flip it, and wait another day or two.
I had an awkward piece at one point that I put in the tub and ran hot water over it for awhile. After a couple rinses and then tossing it in my (then) empty 65g overnight it sank like a rock.
You could always ask a bakery with a large oven to bake it to kill everything. In terms of sinking that piece, it would take a VERY long time to water log it. Best bet it after baking to put it in your tank and simply let it float until it gets water logged.
DONT USE THE CAR WASH! Hand scrub or power wash it and then put it in the tub for a week or so, flipping it occassionally. If you can't use the tub just stuff it in the tank and wait for it to sink.
it looks totally dry. any bacteria that could infect your fish are dead.
none of the bacteria that can cause issues with fish are spore forming. none of the parasites like anchor worm/flukes etc can survive being dried out either.
just give it a quick rinse with a hose to get rid of the dirt and away you go.