Under cut post for stirrup

Verandah post design is for 90mm and for chunkyness I have 135mm. I would like to hide the fixings using a cover of sorts. Can I use a 125mm or 110mm post stirrup and undercut to suit the stirrup which will allow for fixings and cover the butchered bit with a cover?
The cover will be simple glue and screwed timber or if I can work out a better material like cement sheetand or plastic using gal screws to leave air gap from concrete.