Global Industrial has a very large selection of odd larger sizes. Their site is hard to work through but I'm guessing you'll find something close. Start by searching "poly bin".
As far as custom, I looked into it several years back and it was going to be expensive. I found one company in Chicago that would do custom vacuum forming but it did get expensive once the project required custom sheet sizes. At your size, if you included a lip on top, you'd probably be able to work within the less expensive 48" width sheet plastic.
You could always work up your own with fiberglass and a plywood mold. If you didn't need them to "stack and nest" for space saving during shipping or storage they could then be made without a taper and give you a bit more storage.
