Clams are best viewed from above, thus I've always liked the kinds of tanks that have the front panel go up at a diagonal to allow for that. I haven't seen too many people try it though (Ok scratch that, what I mean is I haven't seen anyone I know try it). I guess the tank itself would be hard to find, it's not really a standard thing although it wouldn't be hard to fabricate so you could either get a custom builder or do it yourself (glass or acrylic).
You can see an example of what I'm talking about in the picture on the top of the wetwebmedia tridacnid FAQ page.
http://www.wetwebmedia.com/tridacsysfaqs.htm
If I ever made a "clam specific" tank or refugium one day that's what I'd want to do.
