I've a problem with a set of shelf brackets that I have no idea how to resolve and I'd be very grateful for any help you good people can give.
I have a fireplace that's been boarded up with OSB board (Sterling Board?) apart from a section in which a flame-effect fire sits on the hearth. Just above the area, I have a plate glass shelf that measures about 75cm long by 15cm deep supported by two brackets, one at either end. The bracket type is called a Form Rest when I've looked online in B&Q, but a similar type is sold by Screwfix:
https://www.screwfix.com/p/heavy-duty-shelfit-bracket-polished-chrome-x-70mm-2-pack/60058
I've a fairly heavy cabinet-doored mantel clock sitting on the shelf. In recent weeks, the clock door has opened now and again on its own for no apparent reason. After checking for poltergeists, I've discovered that the shelf is actually sloping downwards with the front lower than the back. I've no doubt now that the slope is causing the clock door to open. The slope is not serious enough for the clock and other stuff to fall off but I'd like to put it right. I've put a spirit level on the shelf and it's virtually horizontal but the spirit level on the OSB shows that it's not 90 degrees vertical to the hearth with the top (against the ceiling) further out into the room than the bottom of the OSB. Undoubtedly, this is why the shelf is sloping.
My question is how can I put this right? I can't see a way this type of bracket could be padded out to take out the slope on the glass. I need a decorative, discreet bracket there so I can't just fit a cheap pair of L-shaped brackets and bend the arm. The glass also has no holes to attach it to that type of bracket.
I would be very grateful for any suggestions. Thank you.