Nevin, cedar lasts a lot longer and more naturally. Also, you didn't say anything about the most important part of the flower box--the bottm. If you have great drainage with screen wire, gravel and a good number and size of drain holes, that will be a big help. Did you use something like Thompson's Waterseal on the outside?? That is nothing more than a wax emulsion that will wash off over time. And I doubt that poly will stick to that now--at least for very long. I don't think that poly will help much for constant water contact on the inside either.
One thing that will increase the longevity is to leave the plants in pots and don't put soil directly into the box. If you leave the plants in pots and let them set on the gravel with the screen wire over the drain holes on the bottom, that's about the best that you can do for now.