I dont think you can find enough place for extra driver in ET1. Probably you can mod existing one by changing resistors.
VG10 quad is great. I dont make batch of parts for them because there are many manufactures and internal dimensions are different. Extra ring (or spacer with high rim around optics) is needed to hold o-ring under glass.
I dont think driver is 23mm itself. Driver is near 18.5-19.0mm and ring is 23mm. Making another ring for 17 or 20mm driver looks more easy than piggybacking another driver.