Hi folks,i`m doing a roof out of plastic corrugated sheeting,the fasteners supplied for this are a bag containing nails and a kind of rubber washer with a cap attached which you click over once nail is fastened.I`m just after these rubber things as i am attaching to steel tubing with bolts.can anyone tell me what these rubber things are called.obviously once on and bolt is in and cap is clicked over it prevents rain water from passing through the hole.The bag of fixings for this product is quite expensive for what they are.The nails arent required so i`m sure theres a site that will sell just the rubbers,If i can find the name for these blasted things.Please help thanks.