The plastic wrap works perfectly for the old screw-down lids, not so much with the new "Universal" one size fits none lids. The problem seems to be that when the lids are wet, they don't have enough friction to hold. And their tapered shape pushes them out even without internal pressure. Of course most brewers are capping the fermenters with a lid they've just pulled out of a bucket of star san.
I think the brick is probably the best idea I've heard so far. It should only be necessary until the "seal" dries out.
NB sells a weighted ring, that's intended for keeping flasks of yeast starter from bobbing about in an ice bath. I have one somewhere, if it's heavy enough it would be perfect, but my gut says you'd need 2 or 3...
I've also hears the suggestion that you can cut-down the width if the new lid, so it fits under the ring of the old lid. Too much hassle for me.