I bought one of these.

Since squirrels attacked my old Nashbar rack trunk, I've had a series of rack trunks, none of which was suitable. They all had soft sides and sagged when I put my tools, etc. in them. I'm going to try this. I just installed it. I'll report after I've lived with it for a few months. On my tour this summer I'm going to tow a BOB trailer, put panniers on the front, and use this to carry jackets, rain gear, sandwiches, etc.

You can find it here.