Maybe suspend them off the bottom, so if they overheat then the pool itself won't get damaged. I don't know if a heater could actually damage the liner if it malfunctioned but rather safe than sorry. On the other hand, plastic heater guards are a thing...
For peace of mind I would definitely use some sort of barrier between the heater and the lining of the pool.Perhaps positioning the heater on a flat rock would suffice.
I haven't used it on a plastic pond but use it when I travel with plastic totes I make a pvc cage for it and drill hoes so water and enter and exit and its never touching anything but the pvc