Henry,
The options I looked at are Phase One's L bracket (comes with hand-strap) and the RRS L bracket and their handstrap, sold separately on their site. The brackets are identical except you don't get the nice laser Phase One engraving.
I have somewhat mixed feelings as I'd like the ability to quickly (and easily) swap strap types, and the design of the wrist strap really precludes that. Kirk L brack has a wider portrait orientation clamp base and should be more stable, but it also blocks the cable release access and/or forces use of a spacer bar. RRS base is smaller in portrait orientation, but does not block the cable release. RRS L bracket does have a slot for a wrist strap. Forgot where I saw it, but you can actually get your strap in rare leather types, ostrich knee, alligator, etc. (while we're spending money, right?)