Old school moulds were certainly made of wood.
What kind of wood? I'm not sure. I *think* if you search hard enough in this forum, you will find some discussion of correct wood species selection for wood moulds.
If you are going to use plastic, I would use High-density polyethylene (HDPE) plastic.
Tap Plastics is good source for that.
I preferr Stainless Steel when I can. When I do buy stainless steel (for my curd cutter....) I buy from here.
Online Metal Store.Specifically, I would stay away from regular steel, or Aluminum (aluminium for our UK cheese folks). The acid in the whey will react and may interact with those metals.
Having never made Limburger, I don't know what kind of mould is used. I could easily imagine having to make a mould for this.
But others have made limburger, They might have a better insight.