Bob reviewed his first year experience with his Nicholson bench in a podcast. In it he talks about a planing stop he built for the center opening in the top that is really neat. It's just a board with notches on one side that fit over the transverse bearers and sits level with the top of the bench. When he turns it over, the board sits on top of the bearers and extends above the bench surface so it can be used as a planing stop. It's a simple but highly functional idea that I'll definitely be incorporating in my bench.
This podcast is interesting in its entirety, so check it out. In addition to confirming that I wanted a Nicholson bench, it convinced me that you can make most of the bench out of construction lumber but the top needs to be a full 2" thick. Christopher Schwarz came to the same conclusion. Yes, you could put in a lot of transverse bearers and get by, but you want your holdfasts to hold anywhere you place the holes for them.