i installed a ladder from railmakers on my oday 20. due to the curve of the transom, i had to cut some spacers on an angle (by trial and error) to fit on the transom under the round mounting plates on the ladder using some oak. inside, i backed it with plywood. i originally tried one flat piece, but ended up using two smaller pieces again due to the curve of the transom. i used 3m adhesive sealant 5200. my ladder doesn't even need a tie down, except when trailering.
couple things to think about:
1. you want a ladder long enough, and positioned so that you have steps in the water when down for easier climbing. (the older you are, the more steps you want in the water : )
2. adequate backing depending on your transom.
3. if you have to tie it down, make sure it's a quick release you can undo in the water.
4. When mounting, make sure the ladder does not get in the way of the rudder when turned completely to one side.
http://railmakers.com/ladders.htm