plasterboard adhesive

I'm about to put up a new ceiling and would like to fit LED strip tape around the edge, about 25cm from the wall. The strip tape will sit behind a silicone diffuser such as eBay item https://www.ebay.co.uk/itm/266101406404 The diffuser has a depth of 8mm, so I'm thinking of using 9.5mm...

I’m in the middle of renovating an ensuite bathroom and I have purchased standard plasterboard for the non-wet areas; XPS boards in shower area. It subsequently dawned on me that perhaps I should have used moisture board instead and this has raised one or two complications. I need to fix a board...