Here is a picture of the RAND KR-2, she's not much bigger than a 3 axis microlight (400kg MTOW) - hard to see and, by the looks of her, hard to see out of:
Very sad, but these things will happen without mandating aids such as FLARM or TCAS/ACAS. I remember a trial done by the MOD's DERA (now QinetiQ) in the 90s that said "see and avoid" below 2000ft was about 99% effective if you look 70% outside and 30% on the instruments - bad news if you are the 1%! I believe that the technology to reduce the risk of this occuring again is now available and it's time for us to invest in it - unpalatable as it is to us hard-pressed GA pilots (ie. Lightweight Mode S + TCAS/ACAS or FLARM).
My condolences to the families involved...
LJ