Navmans don't like being enclosed in anything. The LCD screens get too warm, from being in the sun too long, or not being able to "breathe", and the matrix touch-screens and the internal battery start having a hissy fit.
The other issue you would encounter is, there will be limited GPS signal strength when the Navman is mounted out of LOS (line of sight). External antennas are available, but are unsightly and expensive.
IMHO, its not worth the hassle.
Nigel.