Stainless steel should also work, but might not be compatible with the adhesive (as I presume the sump is acrylic). It might cost a bit but would last forever, although drilling it would be a tad more difficult. Actually, seeing as the bracket should not really touch the water, likely any metal would do. Copper is bad for inverts though and might still contaminate. I would try for a piece of acrylic if available because it should be bondable to the sump and should last forever too. It's easy to drill, shape and trim too. I would even imagine someone has a DIY article for bending acrylic, possibly...
ChakaRaka.