It's the way to go for sure! As Pat Pixley & I can confirm after having to pull the liner out of his repainted Orange Crush Ideal-55 twice, once for the original re-installation, then again in order to remove & reinstall it again to fix a pesky pin-hole leak in his liners' evaporator coils. it was lots of fun!
but with the cardboard trick at least it was relativly easy to reinsert the liner/tank back into the chassis ( which can be tricky with an Ideal unit as it has lots of tubing & fasteners to snag the insulation on the way back into the chassis. but sucess was ours. ha-za!