Big box home stores sell PVC boards that would work. Commonly used for home exterior trim. Expensive, but it doesn't rot.
Example:
https://www.lowes.com/pd/Royal-Building-Products-Actual-0-75-in-x-9-25-in-x-12-ft-Common-PVC-Board/1000422323
It's a piece of PVC board anchored with some ~2" long aluminum right-angle brackets:
Engine bay:
Through the door
Further back view, note there is a U shaped cutout for the steering lines, and it runs all the way back to the transom
I have an '05 Sundeck 200 and I have plenty of stuff crammed in there. It is open to the engine but there is a 1/2 wall that will keep things from sliding into it. There is also a steering cable that runs though, but it's pretty much staitionary.
I think I have a 2 gallon gas can, a plastic...
Also - regarding improving the holeshot while loaded -- I can't recommend Smarttabs enough. My 200 planes in 1/2 the time with like 10% of the bow rise compared to before. I can also stay on plane at 14 mph. Coupled with the 4 blade prop, it will be a holeshot master. Had them on my old boat...
Glad to hear they worked for you! I had them on my last boat too ... it's absolutely amazing that something so simple can make such a dramatic impact. I'll never have a boat without them :)
FWIW I just upgraded to a 4-blade on my 200 Sundeck 260HP MPI
Went from the OE 3 blade SS 14x19 to a 4 blade Al 14x19. Link below. Holeshot significantly improved and overall the boat feels more responsive. I don't care about top end speed -- I think I lost ~4 MPH (50 -> 46 MPH). Very happy...
Here's after modification with a much deeper angle closer to 20-25 degrees. Note that you can get in trouble with lack of adjustability if the actuator mounting is at a vastly different angle but in my case, since the split is about halfway, they average out pretty close to the intended 12...
I just went through this on my 2005 200 Sundeck which I think has the same transom angles as yours -- the plane where the tabs install is nearly vertical, and the actuators mount on ~ a 20 degree from vertical.
According to install instructions, the tabs should rest at ~25 degrees fully down...