You could try a 2
pump system? one for the main res and one for a "catcher" res to push the nutes back to the main. the second being short enough to fit where you need it. It might take some time to get used to pump cycles for the second if you used a pump that would be damaged on a run dry.
Water is not forgiving to square containers. I doubt you will find one for a reasonable price that will fit your needs. Another option could be a small swimming pool? Like a kiddy pool or something? I am not sure what space is available, but They hold up well. You would just have to manufacture a lid for it.
I will do some more looking around for some ideas for you. i might be able to come up with something.