I have a large piece of cedar root of all things that I got from a roadside stand in Florida several years ago. Its about 30" tall and 24" long and oddly enough looks like an angel fish. Its now been in my 150Gal tank for 4 years and if it wasn't bolted to 2 fairly large rocks would still float
Many hardwoodswill not sink. I have some driftwood I picked up off the beach and it won't sink either, so I use pavers or patio tiles to hold them down.
Just setting the tank back up after moving and trying to figure out what I wan to do with it this time. Its 150G tall 48x32x24 so lots of room to work with
not sure about how to do with a large piece like that but i've heard of people boiling their driftwood 30 mins to help it absorb water and sink. i've done it with smaller pieces and it does help.
I soaked it in a kids swimmng pool for about 2 months... its still floated and does to this day. I have it bolted to about 20 pounds of stone. Its a real pain when it comes time to break things down as I have to unbolt it prior to removal from the tank.
I had a large piece that wouldnt sink even with a large piece of slate on the bottom of it. I eventually drilled a few large holes in it and filled it with stainless steel shot(like from a shotgun), then sealed the hole with silicone so that it would stay in there. It sank like a rock after that, but i ended up putting a couple pounds of shot in it. Ive done this on a couple others and found that if you drill your holes strategically you can basically control how the wood sits by putting all the weight at the lowest parts of it(relative to the position you want it to stay at).
BTW-I took out the first piece of wood that i drilled and when i removed the silicone the shot was bone dry. I wish i still ahd that piece because its literally impossible to get more unless you know someone who already has some. It was Brazilian Rosewood, which is illegal to own in the US unless you had it before that law took affect.