Creaky shower tray - help please!

[GH]

I recently installed a stone resin 800 mm quadrant shower tray. Because of the amount of pipes etc under the shower tray area, I opted to use the four legs supplied with the tray. It has been installed onto a chipboard floor. In order to help distribute the load a little better, I screwed down a small piece of hardboard into the chipboard where the base of the four legs would go before standing the shower tray on them. I have also used mastic to bond the tray to the wall and create a seal between the tray and the wall.
The tray appears to be secure; however, on standing in the back corner of the tray, there is a bad creaking noise, and I am sure that I can detect a very slight movement in the tray.
In order to alleviate this problem, I was thinking of tamping into the back corner (under the tray) a 5:1 sand/cement mix, in order to provide the support it clearly needs. Any thoughts? Or how about using that expanding foam that you can get in canisters?
Any help or suggestions would be appreciated. Thank you.

 

[Bahco]

May be better doing away with the legs altogether and bedding the whole tray with cement mix.

 

[Nige F]

I reckon the foam should do .............also, can you confine the area of the foam with some pieces of board etc? before you inject it.......then it`ll expand and press gainst the tray rather than just going everywhere I succesfully used it to fill a 50mm void under a chipboard floor where the floor ran over an old fire hearth........just drilled holes for the nozzle and injected the foam..totally solid.HTH