Stone trays have no flex in them but if they're is any flex in the floor then battens are a good idea - better idea is to re-enforce the floor. Done many ripouts where the tray seal has repeatedly failed because of insufficient support under the tray.
Yes I was thinking that battens would be a good idea as they will hold it much better that just sticking the feet to the floor. I will use battens to hold it in place and the feet to just support the weight across the rest of the tray.
Installed lots of stone resin trays ( often mx elements) with stick on feet without any problems..try to avoid putting any pressure on the feet during the install...dragging/tilting tray. Level the tray and set the height for the plinth from the finished floor. Seal the edges of the tray back to the substrate with a good quality clear adhesive and leave it to cure.
The plinth kits often come with velcro arms and sticky pads to secure the plinth. The adjustable arms screw to the sticky base plates that adhere to the bottom of the tray. Make sure you orientate the plates when you stick them or the arms won't fit properly
