[Arran]

Mike;
I would not use plastic from a milk jug for a spider, first it's polyethylene/polypropylene which resists most glues, second milk jugs are pretty thin and you need some rigidity in a spider, and third the plastic tends to become brittle after a while. I think that the PETE plastic would work better, if it's thick enough. When I made some 45 rpm style spiders I made them out of a card type material, like you would find in a file folder, or a Christmas card. I think that the originals may have been doped with varnish or glue.
Regards
Arran