I really like the Topeak MTX trunk bag. It is insulated and just about the right size for lunch on other items, such as tools, spare tubes, etc. One problem with a lot of trunk bags is the dreaded "flop," that is, they tend to lean to one side or the other after a while. The Topeak bag does not do that. Make sure you get the one that attaches with Velcro straps if you do not have a Topeak rack.
BTW, I have at least 4 trunk bags. The Topeak is not the roomiest but the nicest of the lot and most expensive. I mostly use a Louis Garneau trunk for commuting because it holds more gear, but it flops a little. I also have Banjo Brothers, Performance, Rivendell Lil' Loafer and a no-name brand.