Food grade, in the context of a fermenter, basically means completely smooth, right? That's my understanding of why a bucket from the HBS is food-grade and one from Home Depot is not. I know there could also be issues with the materials itself, but I'm been under the impression that the primary differentiator is how thoroughly they can be sanitized.
My concern with this whole project, whether it's melting/deforming a bucket, or putting something together with fiberglass, or whatever, is that you can't have the little cracks, crevices, nooks, and crannies where nasties like to hide and where sanitizer doesn't reach.