Yup, I marked down those who simply expressed their interest even before I agreed to make any. You are currently #3.
And I would suggest exactly what you are thinking. Stick with the plywood, it is lightweight and strong. I to am concerned about gouging it so I would pick a thin, lightweight (relatively), durable material that could be screwed directly to the plywood structure.
The best part is that most materials can be cut by my laser guy at the same time he cuts the plywood panel (except alum.) So you could have a material of your choice cut and applied to the plywood. The advantage to that is that I can position the plywood down in the box a bit so that the top is still flush.
All that aside, the undercoated plywood on mine is holding up very well and I severely abuse it. As the base model it works very well and is considerably more durable than the worn out carpet.
And, as I mentioned before, your original price range estimate is very close. I am still waiting for the bill from the laser cutter, I have broken down all the hardware prices, and I am still waiting for my final number of hours invested, but it looks like the tailgate box should be right around $200-225. Seems expensive to me, but things add up very quickly. More on that latter, we can save the final price info for the new thread that is coming as soon as it is finished and I have all the numbers in.
Back to the fun tech stuff like what we could cut and attach to the top. I am thinking a thin sheet of black textured plastic could be an upgrade option. I will check with my laser guy to see if he has anything.