I'm going to be putting an NMO-mount dual band (144 and 440 MHz)
antenna in a plastic vehicle roof.

I plan on making a counterpoise using 3 inch wide (or so)
adhesive-backed metal (hvac) tape. I can do an overlapping run of
close to 6 inches wide.

Question for the crowd -- any benefit/detriment for making this
counterpoise as long as possible, or should I stick to odd multiples of
a quarter length (at 2 meters)?

There's more room "behind" the mount point than in front of it, so I
could have a quarter wave section in front of the NMO, and a 3/4 wave
section behind.

I can also do a cross member, but that would be pretty much limited to
a quarter wavelength per side.

Opinions?