I like this design a lot, including the hold downs. I use a pair of Gramercy holdfasts, which are made from steel instead of cast iron, which I've read frequently breaks in this application. If you are having them custom made, you could specify a longer stem, handy for securing short lengths of larger timbers, but then I suppose they would interfere with the tool trays. I have done some reading on them, and it appears that you are correct in choosing a thicker stem. Some people have done research, and found that the deeper the bench surface, the thicker the stem should be to grab effectively.
Anyways, I like that design a lot, and look forward to hearing how they hold up in the long run.
