I ran CVI's list of 1064-nm silica lenses through a la mode looking for a good way to get the cavity beams onto the beat PD. We're looking for a spot with radius somewhere between 80 and 150 um at the PD.

I tried a few optimizations with only one lens in each beam's path, but the results weren't very good; the beam is either too large, too small, or requires excessive path length after the BS. What I've attached is the result of an optimization for two lenses in each beam's path. It gives a 102 um waist a few inches after the last folding mirror, which is nice. The spot radius at the BS is 470 um, which is not too large.