new iMac as monitor for Airlock

I have just bought a new 21.5'' iMac with 4k retina display. I thought I could use it as a monitor for my 2015 Airbook when needed. It turns out the new iMacs do not support Target Display Mode anymore.

Does anyone know of a 3rd party application that can allow me to use the new iMac as a monitor for the Airbook?