Screed or concrete to raise shower tray?

I need to cast a platform to raise a ground floor shower tray to get the right fall. The waste pipe goes below floor level but I don't have enough fall with just that so I need to take the shower tray up. I think I'll need 40mm. Floor is old style so 100mm concrete with 50mm screed. I'll pre wet the floor the day before and use a SBR/OPC slurry to bond the raised base (should I add some sharp sand to slurry?).

The base could be just sharp sand/OPC screed (4:1) or I could add some pea gravel. I can't make up my mind which would be best. I plan to let in a piece of 22mm green chipboard floor where the trap goes (I'll put a smear of silicon over the edges to stop swelling). But 22mm chipboard only leaves 18mm under the the board, too small for 10mm aggregate concrete, unless I chip some of the existing screed away. I could do a 1:3 mix, lay the chipboard on that, then add 1 of pea gravel for the remainder of the base.

Suggestions?

Why not just build a wooden platform?

What does the manufacturer say the tray should be laid on?

There's not enough space. I need 40mm height, say 18mm OSB3 for platform then that is only 22mm under platform to floor and that needs to include a mortar bed for levelling too (floor surface is existing screed and a bit rough). Easiest solution is a cast base of some sort as then it is far simpler to get a level surface, just trowel it off. And to get a solid, well supported surface too, just tamp down to existing solid floor.

Just pour concrete in a wooden cast.
It will last forever.
Don't forget to use an ice cream tub to leave a void where trap will go.