The answer was way too short and correct for my liking
Go for a floating shelf look. Cut out the PB between a pair of noggins. Let in a horizontal peice of 4x2 between them, pop your homemade shelf onto this, another hoizontal peice above this fixed to noggins Make good PB. Two 18mm pieces of ply bonded together, nicely framed will produce a cracking shelf.
Tim .To find the studs ,tap the wall ,You will feel a hollow sound which will change to solid when you hit wood then measure across around 16” which where the next one should be .it is possible when located to cut a section of plasterboard out to fix a middle baton in and then replace the plasterboard to a flush position .