If you want a "cheap" alternative... try this.
Take you a piece of 1-1/2 x 1-1/2 aluminum angle. Cut it into 4 sections of 8" each. then get a piece of 2x6 wood. You might have to use 2 pieces on the bottom to build it up high enough to get up past the lip on the front of your bow, but I don't know what you have for a boat, so it's hard for me to say.
Screw the 2 - 8" peices of 2x6s on top of each other. Screw the aluminum to the bottom piece of wood so that it forms an L to the block of wood. The vertical part will screw to the wood, and the horizontal piece will screw to the boat for attachment (but don't do that yet).
Take another piece of 8" 2x6 to use for your riser which the motor will attach to. Leave enough aluminum hanging down to screw into the bottom blocks of wood, and that piece of aluminum should line up with one of the holes from the other piece of aluminum angle in the bottom piece so that they can be attached to each other. I would suggest using as large of a lag bolt as possible. I would also put some screws at an angle attaching the top to the bottom piece.
Then take molly bolts, or large self tapping screws to attach the bottom mount to your bow - and you should be all set.
Let me know if this doesn't make any sense... I'll try to draw something up