I.ve been using a trolling plate on my Merc 150EFI for several years now. Had to weld reinforcement on several plates since if you even bump the power up slightly it bends. Anyway, I bought a replacement trolling plate last fall (Still in the box) and have now decided to buy a 9.9 Honda kicker due to the price of gas. I agree it's much more expensive than buying a trolling plate, but with the price of gas what it is , I figure i'll save $5.00-$10.00 each time out and even more on the Lake Ontario salmon trips.
I have a brand new Happy Troller trolling plate (large size) and mount, still in the box I'll sell to anyone interested..