I have crawdad jon boat with the trolling motor mounted on the bow. Which causes the back of the boat to fishtail. Does anyone have any suggestions on how to build & mount a rudder on the transom?

I want to build one as well for my pelican.  I think something that fits over/clamps to the back of the boat with an arm coming down to desired depth with a plate mounted to it would work.  I am thinking of buying scrap aluminum from work to make one.  The only poblem I have is I don't have the capability to weld aluminum in my shop.  So I would have to screw it together or just make it from steel then spray galvanize it to keep from rusting.  Here is one that Buster Boats makes, although over priced in my opinion, it might give you some ideas.

http://busterboats.com/asccustompages/products.asp?fav=0&LargePic=ALL&categoryid=3&fpage=Page=&ProductID=12

I made a rudder for my pond prowler out of an old trolling motor i had left.I took the top head off and the motor off the bottom so all i had left was the clamp to attach to the boat and the shaft or pipe.I then took a scrap of 1/4 Plexiglas i had lying around ,i think the size was 8x14 .I then went to the hardware store and bought some 1"conduit hangers,these hangers come with a nut and bolt so it made it simple to attach the Plexiglas.You simply attach the hangers to the old trolling motor shaft and then drill some holes in the Plexiglas to accommodate the nuts and bolts that come with the hangers.I made one this spring and its held up so far. 

I want to build one as well for my pelican. I think something that fits over/clamps to the back of the boat with an arm coming down to desired depth with a plate mounted to it would work. I am thinking of buying scrap aluminum from work to make one. The only poblem I have is I don't have the capability to weld aluminum in my shop. So I would have to screw it together or just make it from steel then spray galvanize it to keep from rusting. Here is one that Buster Boats makes, although over priced in my opinion, it might give you some ideas.

http://busterboats.com/asccustompages/products.asp?fav=0&LargePic=ALL&categoryid=3&fpage=Page=&ProductID=12

I bought one of these for my pond prowler type boat. If you decide not not make one yourself these are great.

I made a rudder for my pond prowler out of an old trolling motor i had left.I took the top head off and the motor off the bottom so all i had left was the clamp to attach to the boat and the shaft or pipe.I then took a scrap of 1/4 Plexiglas i had lying around ,i think the size was 8x14 .I then went to the hardware store and bought some 1"conduit hangers,these hangers come with a nut and bolt so it made it simple to attach the Plexiglas.You simply attach the hangers to the old trolling motor shaft and then drill some holes in the Plexiglas to accommodate the nuts and bolts that come with the hangers.I made one this spring and its held up so far.

I did the same thing with an old trolling motor and a scrap piece of old stiff plastic.  I am not sure I made the actual rudder big enough. How big is your plastic rudder, mine is about 4 x 4, I think I need to make it bigger. 

Go bigger, like the size of a canoe paddle.I went with the biggest scrap of 1/4 Plexiglas i had lying around, it happened to be 8x16 and its worked perfect all year.