I have a V-Hull and would like to mount a bow trolling motor but the plate is too small. I'm thinking about making a base plate using 3/4" plywood wood and bolting it to the rails and mounting the motor to the plywood. Does this sound ok or any other suggestions.

Better suggestion, go to the sheet metal shop with a cardboard model of the size mounting deck you need and have them cut one out of approx. 1/8" aluminum. You can have it welded on, you can pop-rivet it on, or use screws. You will never have to replace it.

Is 1/8" thick enough ?

Most heavy duty aluminum boats are .100 guage, a lot are .072 guage and 1/8" is .125, so yes it is enough.

Thanks....The stuff at the local hardware store was 1/8" but has to much  flex in it...

Thanks again for your help.

http://en.wikipedia.org/wiki/Sheet_metal_gauge

The thickness numbers I posted before were in thousands of inches  like .125"

You can probably find the specifiations for your boat that indicates your hull thickness.

A buddy works for the Highway Dept and tells me they throw away the old highway signs that are damaged. He can get me one. Would this be ok to use for a mounting plate?

I sold this aluminum boat I had a couple months ago but here's the little deck extension I had for it.  IIRC, it was 1/2" ply I coated with helmsman urethane and then carpeted.  I had some 2x4 framing underneath to prevent flex.  The deck actually extended out over the gunwale lip.  

A buddy works for the Highway Dept and tells me they throw away the old highway signs that are damaged. He can get me one. Would this be ok to use for a mounting plate?

I would think it would be fine.  Worse case double it up if your getting them for free.