I think your box design will work.
We use a .625 net volume (about .675 gross internal) side-firing sealed enclosure built from 5/8-inch stock. It fits in under the driverís console and over the floor hump and extends to the bow surface. Itís a simple rectangular shape. You can insert the box into position and barely clear the underdash steering mechanism. This requires zero modification to your Moomba. Bracketry is a little complex. There are a number of good 10-inch woofers that will fit this displacement including Polk dB or MM, JL Audio W1 or W3 or W6, Kicker, etc.
A Boston Acoustics G2, G3, or G5 10-inch sub (facing exterior) with a 10-inch passive radiator (facing interior) works in a 0.5 cu. ft. 3/4-inch enclosure.
Note, the Moomba and Supra consoles look similar, but are very different inside.