Someone asked for this, and it's been done for a while, I just never did a decent field testing of it.
Specs
1.5" L aluminium from Lowes (6061 I believe)
NMO L bracket w/ NMO mount
I'm not sure of how good of a ground plane this is, but I was able to reach at least ~20mi on mountainous terrain using 45W and the cheapest VHF 1/4 wave whip I could find ($9.95 + s/h)
This secures to the rails using a piece of sheet aluminium and a nut jb-welded to the back, and a 1/4-20 bolt. Yes, the bolt is rusted, yes, I'm replacing it, hopefully this week. Alternatively you can use a square spring nut from the electrical hardware section @ Home Depot, except you have to cut a good chunk of it to fit on the side rails, or attach something to it so you can mount it lengthwise and prevent it from popping out.
Antenna mount
Antenna mount
Antenna mount