After studying the CREE LED data, and the reference materials I found, it looks like an array of 12 of the 100 lumen CREE LEDs will do what I want, without lenses, with the fixture lowered back down to the top of the tank. Here is the layout:
I think it takes about 4 overlapping LED light cones to get to the 50 micromol PAR that I want, but that is a very crude guesstimate. Twelve LEDs may be overkill, but at about $3.50 each, the price is right. This whole set up should get close to my $150 goal too.