Checked out your thread, nice work.
My plan was to wax the plug and melt the foam with acetone which was recommended to me. My duct is "irregular" like yours and the plug won't simply just slip out. I'll definitely have to cut my bulkheads just to install intake duct
I'll put a bit of thought into your idea about making a single slit in the duct to extract the plug without destroying it. Might be a good idea in case I ever want to make a 2nd one...