my thoughts are
go for foaming pu glue cut a block to within 2mm off the open void size and perhaps 6" long
coat the surface with a 1mm bead on all surfaces in a zig zag pattern
push the block in and fully seal the open end surface with plastic to stop foaming outside on the surface
after about a day remove the plastic an clean the face then use dowels
in my humble opinion no doweled and dismantleable options will be solid enough in use
if you can add batons to the back for strength then fine or batons to lean on the wall to stop flexing you should be ok
go for foaming pu glue cut a block to within 2mm off the open void size and perhaps 6" long
coat the surface with a 1mm bead on all surfaces in a zig zag pattern
push the block in and fully seal the open end surface with plastic to stop foaming outside on the surface
after about a day remove the plastic an clean the face then use dowels
in my humble opinion no doweled and dismantleable options will be solid enough in use
if you can add batons to the back for strength then fine or batons to lean on the wall to stop flexing you should be ok