NorthSouth :
I wonder if one practical measure would be to require controllers to inform IFR traffic when it's about to enter Class E airspace
Indeed that would help a it, but only a bit as you and I know that any modern airliner windows were not designed with spotting VFR traffic in mind.
A glider in a thermal is at 50-60 Kts and as it turns tightly , easily visible but only 1/2 of the time ..
. Any jet coming at it at 250 Kts with current or descending gradients has very little chances of spotting it in time to make effective avoiding action .
A modern glider in transition is around 150 Kts and sleek per definition so extremely difficult to spot if opposite, especially regarding sun position.
Only an electronic solution will work, FLARM is best and/or Mode S Transponder .
But FLARM is not certified ( and won't be since it uses a not released frequency ) and therefore cannot be legally fitted to airliners ( although I know some who have it )