It might be easier to just fill the extra area with dirt. As long as you have good soil (like Mel's mix) on top, you can use ordinary dirt to fill in the lower part. You won't have to use as many cinderblocks and you won't need plywood.

You could also fill the extra space underneath with chunks of waste timber to get a hugelkultur effect going, and top it with a richer mix.

From my experience, the typical plywood sheeting panels rot in 3 months unless treated. I just don't see it as a good idea. What I would if in your case is to use a sheet of treated wood, then place a heavy duty tarp over to protect it from the soil, this will eliminate your concern with toxin leaching (unless you're concerned with using plastic as well). You want to have some concrete block in the middle to support the weight of all the soil unless you're gonna fill the bottom with sand or something. If you are going to fill the bottom, make sure there are no gaps or somehow promotes ants nesting, which will be a big problem.

I know the tarp idea works because I made a concrete block raised bed on top of stone pavements, I had one layer of heavy duty tarp in between. After one year I had to relocate the raised bed (using a dolley made it easy to transport the blocks, but only one block at a time otherwise it's too heavy). The tarp underneath was like new. Tree roots can puncture the tarp, but not annual vegetables roots growing for just one season.

If you have back trouble however, you really should reconsider using concrete blocks and sand which are both very heavy. Unless you want to do it one time and is sure it will be a permanent structure.

Consider building a stepping raised bed on a slope, if that will suffice.

Another option is "corrugated steel panels raised bed" since they tends to be tall, I suggest looking at some of the designs. It seems a bit more expensive, but will last a long time. You may have to build a wood frame so it's comfortable to sit one and work with, but it's easy. Although will likely need a circular saw to help with the job.

Just being picky, cinderblocks are applied more for blocks that may have fly ash and very toxic, you probably mean "concrete blocks". Even with concrete blocks I think if they are made from these factory can have some small amount of toxin just from the manufacturing, though I'm not concerned. The other consideration with concrete blocks is it's made of lime so some people have a concern with alkalinity. My veggies grows well in it though, but I didn't have much luck with tomatoes, though I'm sure there are people who have.

If you have a twin size metal bed frame that have wire grid, perhaps putting wooden raised bed on top?