Green chipboard should be fine if well supported. You got a big heavy tray there. 15mm or so of sand cement. If it has recessed back, then maybe more to start. Don't let the tray be supported by the edges only.
You'll need two people to man handle that thing into place. The mix shouldn't be too wet. Comb it out so there's a bunch of peaks and valleys. Make sure it is level in all directions. Sometimes I stand on it and wiggle it back and forth to push down the high bits but on something this heavy, you can probably just wiggle it with a bit of pressure on the high bits. Too dry a mix and you'll struggle to level it.
If this doesn't work first go, lift it, see where the hollows are and bits that are well packed down, move the packed down bits to the hollows, comb it out again and pop the tray in for another go. This is the advantage of sand cement. You'll be unlikely to lift it if you're using adhesive, glues, silicon and the like. Check levels along edges and diagonally. And also many trays are not totally flat along the edges which doesn't help.
Modern big trays with a low profile have minimal slopes so once in, check the falls in the tray to ensure that it goes downhill to the drain. Sometimes the middle drains to the edges first before the edges slope downhill. A water test is best to see it drain. You want to avoid puddles but don't get confused between a puddle and surface tension on these small slopes.
Hope that helps and isn't too confusing. Watch a Youtube video. There must be one out there some place.