if you find a not only acceptable but satisfying solution that is even worth its money and is affordable millions of iMac owners and potential future iMac owners will thank you as most of those cling film stuff known to menkind that gets glued on the glass panel has downsides which you can see
the most ideal and cheapest way is to remove the glas panel and cut out a black piece of cardbord to give the impression of the glas panel still in place , as the actual display offers less glare