GMM wrote:

Installing a back to wall bog and having to custom build the boxing in
as the easy way (buying a cabinet) doesn't fit the space. The bog and
sink will be side by side on this boxing and I'll need to provide for
potential access to the cistern and (just in case) the plumbing under
the sink.

Stick tiles with flexible adhesive onto a loose panel plywood, then use
strong magnets to hold that in place?