I use a flex plate with Buildtac to make removing prints much easier with less risk of damaging them (or me!). It works really well but I had to remove the glass bed plate to fit it as the DaVinci Jr doesn't allow much in the way of z-offset adjustment. Even with the flex-plate and magnetic base installed in place of the glass the extruder nozzle was still quite tight to the Buildtac in places and it wasn't possible to level the build surface as much as I'd have liked.
So recently I decided to make a riser for the bed-height sensor switch so I could reinstall the glass plate and stick the flex plate onto it. That's worked out really well as I can set the z-offset easily and the flex plate is much more level. It's really simple to install: just take out two bolts (replace them with M3 ones a few millimetres longer) and reinstall it with the riser underneath. Then adjust the z-offset and you're ready to go.
In case you have any problems the OpenSCAD file is included here so you can make minor adjustments and fine-tune the design to your machine.
I printed these at slow speed, thick shells and 0.2mm layer height.