I have a rubber pad and mount the motor to that directly on the transom.
http://www.cabelas.com/jack-plates-moeller-transom-pad-1.shtml?type=product&WT.tsrc=CSE&WT.mc_id=GoogleBaseUSA&WT.z_mc_id1=737062&rid=40&mr:trackingCode=AF097452-F5D2-DF11-82EF-001B21631C34&mr:referralID=NA
When I trailer, I don't trust the C clamps to keep my motor on the transom on the highway. I didn't like the idea of having the motor in my truck or in the cockpit of the boat so...
I took 3, 1X12 planks and used the trailer jack as an attachment point. I screwed 3 planks together which were cut like stairs so that at the top the motor only mounts to a single plank the plank opposite the trailer jack which for me is the port/drivers side. This plank is the longest of the 3 and extends below the trailer and trailer jack bracket by an 1-2 inches. I used the jack bracket as a template and marked out my 4 bolt holes. I re-asssembled the jack with longer bolts sandwiched between the longest board.
I screwed the shorter and shortest boards using 2 in screws and used several. I took the top bolts out of the jack and marked the hole location thru the longest board. My jack bolts sit flush w/ the top of the trailer, so I couldn't drill holes for them I ended up having to notch the bottom to allow the boards to sit flush on the trailer.
I screwed all 3 planks together tightened up the trailer jack bolts and it was rock solid.
Lastly I added a 2X4 between the back of the planks and winch to act as a mast crutch. I cut up 2 ratchet straps one to hold the mast down and the second to hold the motor tight to the planks. I was going to attach this to the winch attachment but I saw no need.
I also built an upside down T that sits against the transom and is secured with a line thru the transom and back around the top. I used those blue ladder foam pads to protect the transom and sole of the cockpit. on top i added a V made from metal plates covered in some scrap garden hose to protect the mast. It's super solid.
Honestly, I'm overly satisfied with this mod. I didn't have any set plans just started out with a beer and ended up with this. I trailered every weekend for the last half of the summer and the mast never budged,the spreaders lay open on the cuddy and it simple to raise the mast single handed.
The only thing i would change is the height of the T in the transom so that when the mast is at the tabernackle it is parallel to the tabernackle hinge to make for a true single hand raise.
Total cost was under $40, the biggest expense were the 4 grade 8 bolts. lumber was cheap and the rest was picked from the shed and basement.
If you want I'll take some pics tonight and link them here.[/url]