On Sun, 2 Jun 2019 21:41:43 +0100
ss wrote:

I havent ordered tray yet.
Ideally I want a 1200 x 800 tray.
Unfortunately the waste connection would be directly above a joist.
A longer 1300 tray all appear to have the waste in the middle of the
tray and would need to attempt access to connect from the adjoining
room so trying to avoid that. I am trying to achieve a sort of
walk-in so a shorter tray I am trying to avoid.

Any thoughts on how to solve this I am reasonable at DIY, plumber
will make connections though.
See image.

You could custom make to your own requirements in concrete and/or GRP.