I came across a pulley in a local electronics store that I will be using as an idler for my marauder. It's 3 1/2" dia. with a 3/8 hole in the center. Made out of what seems to be a pretty tough plastic. It runs on bearings but not of the best quality. But the price is right, $3.00. I just hope it is a large enough diameter.

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As you can see the chain slides through quite nicely, but the bottom of the groove is rounded so I may open it up a little with a square file to make sure it doesn't grab the chain.