- While it is possible to affix supports to the walls, I would very strongly prefer not to do so, as it would mean drilling into the attic-space behind the alcove, which is drafty and very cold! The previous owners had such holes and the difference when we sealed them was immediate and delightful. I am quite desperate to avoid any openings into that space.
I've attached some photos I took, one of the vanity in front of the alcove, and the second of the alcove on the other side which is of roughly the same dimensions and a rough guide of the shape of structure I had planned to put there, which weighs 60lb.
Would this sort of shape be sufficient to add counter-weight to the mirror? The bottom of this is very heavy, and there are 2 additional timbers behind to add further weight. The only problem with this approach is that there is a "lip" of sorts at the bottom of the alcove which would have to be bridged somehow.
I was considering building two long legs for the mirror to be screwed to (almost like headboard legs) and attaching these to the structure pictured. Would this be workable, do you think? If this would work, it would tick all the boxes.
I can see from the Ikea instructions that the "Frame" is mounted by only 2 holes towards the top. Hence it is designed to "hang" on a wall.
(I can't see any need to create another frame into which the Ikea frame would be placed!)
If not screwed directly to a masonry wall, in a timber framed building it would be unlikely that suitable timbers would be in the "right" positions and, therefore, a "back-plate" for the frame would be necessary and this could be affixed to the building timbers wherever required.
Because of this, I am perplexed by your desire not to drill holes because of drafts unless the holes are sealed.
Surely any holes would be drilled into the wooden structure behind the drywall and not just the drywall itself! When the back-plate is then screwed into these holes they would be sealed. If necessary, caulking compound could be used, although that may make removal difficult.
(Actually, it would be necessary to locate the position of the wall studs, position the back-plate where required and mark the position of the studs behind it on the back-plate, drill holes in the back-plate to act as a template, then drill through these holes into the studs and affix the back-plate to them.)
Any such back-plate need only be long enough to bridge the two studs to which it is attached and thick enough adequately to hold the screws (probably) provided by Ikea.
A short length of 2" x 1" would be sufficient, although this would allow the mirror to "swing", which you may not want. Hence, a back-plate of 3/4" plywood may be required (Do not use MDF)
"building two long legs for the mirror to be screwed to (almost like headboard legs) and attaching these to the structure pictured." is the equivalent of using a back-plate screwed to the wall in that the Ikea frame will sit "proud" of the wall by the thickness of the either the back-plate or the "legs". (You will need at least one horizontal member "lapped" between the "legs" and at the correct height so that the screws in the Ikea frame can "pick-up" this member.) If the "legs" reach the floor and are jammed/secured between the vanity unit and the wall, it will not be possible for the mirror to tilt forward without moving the vanity unit forward so that raises the question of whether and how the vanity unit is secured.
If the base of the alcove is timber - and is solid enough (at least, 5/8") - it would be possible to screw the insert unit which you propose to this base at the rear. This would probably be more effective than a 60 pound mass and, when combined with the "legs" against the wall and the securing of the "legs" in position by the vanity, should prevent any forward movement.
