I've been musing on the idea of a heavier duty lid than tin foil for this and I wonder if perhaps an appropriatly sized lid from a can would do...of course you would need to use a side cut can opener like a Pampered Chef can opener.
I wonder if you might be able to use some of the aluminum from a disposable turkey pan? It would be sturdier than just aluminum foil, but less so than a can opener. Similar with weight, I imagine. The best thing might be that you could have multiple tries, and make it bigger than the opening then notch it so that it would "close" tighter if you wanted such a thing.
Just a thought! (and it's a bit late... so this might not make sense in the morning. <img src="/forums/images/graemlins/blush.gif" alt="" /> )