search for "shaft collar". Narrow by 3.5" and stainless steel. They have both a single piece and a two piece in 316 for around $200. Personally I would go with the two piece split collar, even over something with a thru bolt. I locktite fastened collar like that is very secure and quite strong, and can be adjusted to take up play. It will only work well if the shaft is really 3.5" or within a couple of thou of that. Might be, if it was turned, if it is piece of pipe it might not be very close at all.

I'd offer to machine what you need, but I'm supposed to be on my way to the Bahamas....