Bedding down a low profile shower tray

[GaryMo]

Im looking to bed down a 1400x700 low profile shower tray on a 18mm hardwood ply floor which runs out by around 20mm from back to front.
I’ve read that a mortar mix can be used but wondering how a flexible rapid set cement based tile adhesive would work? I worry about cracking of the mortar mix over 20mm.
What do you all use and what’s the best way to lose 20mm?
Thanks in advance.

 

[Nige F]

Mortar mix is the recommended bed but you won't get 20.mm to nothing. What I've done is remove the flooring and create a board between the joists at a lower level- screwed to battens on the joist sides. That way you'll get a decent level bed of mortar