I want to cut thin lines across the fins so there are loads of little 'pins' sticking up, then I want to have it so water enters from the centre and flows outwards in all directions (see BB2K's 'RADIUS' thread)...
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That top is the right idea, it just wants to be more 'gradual/linear' (can't think of the right word!), so the circle opening
flows into the 'slot shape'. Imagine getting a copper tube and squashing the bottom into a slot then stretching it to the right width so there's a smooth transition all along the change of shape
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Just open it out/smooth it with a needle file and you'll be good to go!...