Agree with Zacs. If you have the tools - a small woodsaw, hammer, no nails glue and/or nails, measuring tape, square etc and some 1" square battenwood and plywood, it's easy. But that is up to your skills and determination (and how neat you want it to look).
Personally I would ask around friends and see if someone would do it or help you.
Often with a batten frame covered in plasterboard... then skimmed (pink plaster)
To avoid plastering, use "Contiboard" for kitchens and bathrooms. (Melamine covered chipboard). Screw using cup washers or little plastic things to make the screws a little less unattractive.
Or, batten frame, then plywood with a small strip of wood to hide the rough cut edge...... then polyfilla.
Frame plus MDF and sand the edge...... polyfilla and paint.
Also, B&Q etc stock little plastic corner pieces that will finish a corner nicely.
I did my boxing in some years ago and it took 3 kitchen refitters to shift it to get to the pipes, so I think I did a pretty good job. My kids jump all over my boxing in the bathroom too. No nails glue, plywood and 1" battenwood is the way to go, cheap, quick and, hopefully, smart.
There are so many exposed pipes behind my loo, I would have no idea where to start - some vertical, some horizontal, then the soil pipe itself - oh and a stopcock too.
I have just stopped putting the lighton, Landlord doesn't think it is a problem.
I like the idea of the ones you buy and click together. Can anyone provide a link please? I'm fine with painting and decorating, but I've not tried this before :) TIA