FUNNY you should ask. Mine fell off this summer, with a little help from my pals 4yr old. The letters (47$) have hot glue adhesive on the back, and are supposed to be applied with an iron from the BACK side. I didn't feel like going through all that, so I used contact cement. They have stayed on for 2 mos. so far. I applied it to the backs of the letters and to the seat with a toothpick, then stuck them on the seat. They came out OK, I could have gotten them a little straighter. Definitely keep the old ones. I got the new ones through Toyota via Atlanta Marine, it took a couple of tries.
Of course, I took pics of it all and will post them when I get home.