Buy a cheap camping sleeping "roll mat" from Halfords etc, and use this, cut to size, to roll round the full exposed length of ducting. Try not to leave any gaps, and use two layers if you can manage this.
Cover the whole lot with waterproof "gaffer tape" (Halfords again). The silver coloured stuff seems to be the best, if you can find it! Finish off with several "Ty-wraps", but not too tight.
DaveMac