I live in the UK so the terminology may be different, but if you want a pillarless look, the idea about making the quarter windows slide in is a good one. But, if you want a good weather seal too, cut a strip of glass and attach it to the inside leading edge of the quarter glass using a silicone type adhesive for fish tanks. The good thing about this adhesive is that it will not discolour. Then, attach a thin strip of rubber to the face of the strip of glass so that when the door is closed, the door glass sits against it giving a good seal. Hope this helps.
Gareth.