For that annoying peice of driftwood that just won't stay down by it's self, lead weights from a fishing tackle store, some nylon string and a hand held staple gun.
Drill a hole in a piece of slate then run stainless screw thru the slate into wood and cover the screw with silicone. Then bury under substrate and you should be ok
it is not exactly a giant piece of wood, just put it in a bucket of water for a few days, then it will be water logged. no screws, no glue, no lead, no butter knives - just the wood at the bottom of the tank.
Lead won't leech or react with the water... as long as the fish aren't chewing on it, it probably won't hurt them.
The staples however will rust. I considered something similar - drill a sizeable cavity in the log and fill with rinsed lead shot, then seal with hot glue or silicone.
metals dont get to build up to toxic levels in lakes and such since they are in a huge volume of water but the home aquarium doesnt even compare to a lake and aquariums just cycle the same water allowing toxins to build up which is why we need to do water changes to lower nitrates and heavy metals are more toxic then nitrates.