Had a go at making the gutter drain tonight. It came out ok, well good enough for my old van.
First cut a 22mm long slot with a pencil die grinder, similar to a dremmel.
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Then made a die to form the shqpe, initially was going to clamp the two pieces with a bolt however ended up using a vice to clamp the die pieces together to get enough force.
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NogE I'm using zincanneal which is harder to work than body metal so this method may work for you using a decent G clamp. I can give it a go with a clamp if I have a chance however just wanted to get these done for now.
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