Please note that this process will add a third party TurnKey repo that I personally manage. I had to do that as I do not have direct access to the offical TurnKey repos and we need updated packages for a Stetch base.
Starting point: v14.2 TKLDev running in a VM (or bare metal if you prefer...), logged into terminal as root.
# checkout appropriate branch of common and core
cd common
git fetch origin # assumes you stil have turnkey as origin - can be skipped on fresh v14.2 install
git checkout -t origin/stretch-dev
cd core
git fetch origin # assumes you stil have turnkey as origin - can be skipped on fresh v14.2 install
git checkout -t origin/stretch-dev
# build new ISO
export RELEASE=debian/stretch
export DEV_TEST=y
make clean
make
Since I couldn't find a TurnKey bootstrap for stretch, I took a chance and just renamed the jessie bootstrap. I figured that all it needed was a bootable system and that it would automatically be upgraded to stretch during the build. It seems to have worked as the core build completed.