Inductive sensors work just fine with MakerFarm printers. This is the one that I use and it works fine through 3mm borosilicate glass. It has a sensing distance of 8mm, which works out to about 3.2mm to 3.6mm when you account for the target correction factor of the MakerFarm construction.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Target correction factors http://www.ab.com/en/epub/catalogs/1...y-Sensors.html