This is a shallow, lightweight resin filled tray going on an 18mm ply floor. Assume the sand / cement mix is not used on a lightweight tray (?) I've received suggestions ranging from drywall adhesive, 'no nails' and criss-cross 6mm beads of silicone 200mm apart. I need to have sufficient thickness to compensate for 3mm out-of-level. Thank you for any responses.