I was visiting my local metal shop today, and I told him I need a cheap 100% effective bypass draft control for 6" stove pipe. He recommended I try a blast gate. They're "used to open and close branch ducts in a dust extraction vacuum system allowing air to be diverted only to machines that are operating, thus conserving air volume."
Its actually cast aluminum with a sliding galvanized steel blade with a locking screw. Since it will be open on start up, he didn't think the galvanized metal would be a problem and when its closed its out of the direct exhaust flow. I can dissemble it and easily replace the blade with heavier gauge stainless if necessary. Its also relatively light and fits inside 6" stove pipe well. A 6" is ~$15. They make sizes from 2” up to 24”.