I have a bunch of non-standard size lumber. They are close to common 2" x 4" dimensions and anywhere from 8' to 10' in length. (They can't really be used as 2" x 4"s unfortunately as the dimensions vary too much. They are salvaged from packaging crates, FYI.)

I thought I would go ahead and use them to create a stake bed or two and then sink it in my pond. I also have some scurvy CMU's that I will use to weight the wooden stake beds. My questions are:

How many CMU's per 2" x 4", roughly, will I need to keep this apparatus from floating on me?

Assuming an 8' to 10' height, how close should I space the vertical 2" x 4"s?

TIA for any advice. I plan on building these and dragging them out on the ice here in the next couple of weeks and then letting them fall through the ice as it melts.