Just having a quick looks around, found this.
http://mykin.com/rubber-...mical-resistance-chart-5And this from sodium hydroxide safety sheet.
Skin Protection: Wear chemical protective clothing e.g. gloves, aprons, boots. Suitable materials include: butyl rubber, natural rubber, neoprene rubber, nitrile rubber, polyethylene, polyvinyl chloride, Viton®, Viton®/butyl rubber, Barrier® (PE/PA/PE), Silver Shield/4H® (PE/EVAL/PE), Trellchem® HPS, Trellchem® VPS, Tychem® SL (Saranex™), Tychem® BR/LV, Tychem® Responder, Tychem® TK.
So most kinds of rubber look to be okay. Will probably chop up a couple of bits of the material do a little test with lye and naptha just to be sure but looks pretty safe from here.
Will report back.