filling 10mm gap

[petey]

Hi folks.

Fitting a 1600 shower-bath in a 1625 gap, and taking into account the tiles and adhesive, the bath will leave an 18mm gap (give or take).
I cant really dot/dab plasterboard to either sidewall to make the gap up, as one has a door, and the other has a window.

The rest of the bathroom is not being tiled, but I figured I could tile a small area around the 'head-rest' end of the bath (imagine like a splashback on sink) and think that would look okay, but the tiles will only account for 10mm, not the 20mm I need.

Could I cut a piece of hardibacker the same size as splashback area, and tile onto that? Trouble is - how do I fix the hardibacker to the wall, and how do I deal with the 10mm return-edge on it ?


Any advice ?
Thanks.

 

[Colour Republic]

once tiled it would leave a 5mm gap at each end so this could just be siliconed.

or use 9.5mm plaster board or hardibacker above one end of the bath, then use a thicker trim it cover both the tile and backerboard

or use 6mm hardibacker on both ends of the bath and again use thicker trim to cover at both ends