Other Best material for building mold flange on plug

So I have a plug made I want to take a mold from. What is the best material to use for making a flange on the plug? I need something that will match contours of the part I'm making the mold from. (ride plate to be exact). It just needs to be strong enough to hold pva, tooling gel and a little glass. Suggestions?
 
aluminum flashing from lowes/home depot and a hot glue gun! (comes in several widths) perfect for bagging flange
really easy to form and shape, for complex shapes you just work in small sections and work your way around the part.
also some clay from hobby town (arts craft store) that stays soft works great to fill hard voids like the gap from part to flange.
 
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