I have seen repeated references to duct tape in the forum. The soft shiny cheap silver duct tape that is "sold everywhere" is actually not a very good sealer. It may hold at low pressures or depressions when it is freshly applied, but it doesn't hold up in the long run. A better tape for sealing on the flow bench is foil tape. I'm adding a link for one common type, but there are several brands out there that are similar or equal. The foil tape is commonly found at distributors that cater to the HVAC and Insulation trades.