I weighted down my driftwood using the decor and it took around 2-3 weeks to sink on its own (it's around 45-50cm long to give you an idea). it is still slightly buoyant but not enough to move with the current.
I used petrified driftwood since I can get it easily but that’s not likely available to you.
I like flukestone’s idea of using decor, that would work.
You could get a metal trash can to boil the wood if you want to do some extra work?