Silicon for gap seal

[skeetstar]

Folks, I usually use a silicon bead for small gaps of 1 to 3mm, but I've been asked to seal a shower tray that has sunk by about 6mm. First approach will be to try and jack the tray up to close the gap, if I can't do that, is it OK to use silicon the seal off a 6mm gap?
Is there a better way? Is there a useful hack that I don't know about.

In section the gap will be 6mm vertical from top face of tray to bottom edge of tile, and horizontal from front face of tile back to plaster, depth being whatever the thickness of the tile is, plus the thickness of the tile adhesive.

Grateful for any advice.

 

[Pedronicus]

You don't say what material the tray is and whether it is installed on a suspended or solid floor. TBH I would be trying to find out why the tray sank in the first place rather than just fill the gap (which could get bigger) and then try to rectify the sinking rather than bodge the job.