Howdy scubyfan,
Not sure if your moniker is 'scuba', or 'scooby' related...but just in case it's the latter, maybe I've got a 'scooby-treat' for ya.
I've done a wee bit of sculpting myself, and I have never really used Balsa that much. What I have used successfully is Basswood. It's almost identical to Balsa, except, the wood is denser, is much easier to carve, and resists 'tear-out' a bit more than Balsa. Now, I've never tried to etch panel lines in it, so I'm not
entirely sure it will work, but it's certainly worth a try. Just stop by your LHS and pick up a small piece and try to lay your line.
And please, if you do, let us know if it works.
Also, if money is no object, you can use REN shape.
hth.
Tread.
....now, as to the rivets...............sheeeeeesh.......
Since this project is obviously 'Secret'.....can you at least tell us it's scale? I mean, "50 inches" tells me something, but certainly not enough to go hogwild with the input.....