Your Miata would fit an an all-Aluminum UTV trailer. Polaris sells a nice one, as do many local trailer builders.
I've had all three materials for the decks. I like AL the best because it doesn't rust. I'd pick steel 2nd. I don't like wood because the point loading on the deck causes those spots to soften first, and eventually a wheel will break through. Plus wet wood has the worst traction, and the paint just doesn't last long.
For balancing, you could a 4x4 spanning between a block and a bathroom scale, with the tongue at the mid point. Then double the scale reading .
The Miata is a well balanced car, so loading it just barely forward of center (mid point of car vs axle center) should pretty much be perfect. You can go as low as 8% tongue weight with a car trailer, since the moment of inertia is less than a TT. Kinda like boats.