Pictures show a mat board mockup of a 25mm thick housing. This box is a conservative minimum size for unstacked components with a sloped front. Final would have curved top that matches contour of dash. Sides would be sloped out a few degrees, with corner rounds. Housing would be larger in hgt and wdt to add these features.

Upper Right: Unit sitting on dash. For comparison.
UpperLeft: held where display would be if vertical and hanging over front of dash. Top of display would be even with top of housing if housing were held flat as in upper right.
Lower Left: Wedged under dash, a steel box would give enough to fit. Sideways because it just bearly fits between paint and, I'm not sure what, I'll call it the firewall. The package would have to be no more than ~15mm thick to fit and no longer than it is wide. This pretty mutch puts paid to the idea of tranny mount.
Lower right: Tranny left side mount. Would interfere with fwd seat movement. Right side mount (not shown) is undesirable as the hump is already biased toward the passenger.

Still exploring, but the remote mount is looking better right now