I have been holding down driftwood for decades using 4 items.
1. A piece of ceramic tile- size and shape based on the wood to which it will be attached.
2. A glass and tile drill bit to make a hole in the tile and another for wood to drill a pilot hole in the wood.
3. Stainless steel screws. I have them if an assortment of sizes and lengths.
4. Nylon spacers. These are small cyclenders with a drill hole through the center. These allow me to attach the tile to the wood and then to bury the tile in the substrate and make it disappear. It also means very little of the wood itself will be buried.
Early on I was using a lot of cyprus wood which when dry floats better than a life preserver. it had to be weighted or it floated. So the above was the solution. Over the years as I got more into plecos I needed a lot more wood and decorative value was less then the shape of the wood giving lots of hiding places. I wanted wood to sink right away, I switched to Malaysian wood.
I have a number of pieces i wanted to be vertically oriented like trees. But this was not possible unless I had a base on the bottom so I could keep the wood vertical. Tiles, SS screws and spacers as well as an assortment of drill bits did the trick. Almost all of the wood in this 150 gal. tank has a tiled attached to the bottom. Some have spacers and some do not.
Over time pretty much all wood becomes water logged and will sink. But it can take some time for this to happen. All of the wood in the pic above sank on day 1 save a huge log I have had for close to 20 years. Initially, I needed a square foot tile that was almost an inch thick. The wood has sunk on its own since I upgraded the original 75 gal. to the 150. But that was after years in the 75.
edited to correct many typos and spelling problems. boy did I origasnnly type this post too fast, sorry.