I got sick of wiggling into my bivi bags so I decided to get the sewing machine out and add some zips to them. I never use them on there own without a tarp so I didn't need to use waterproof zips and seam sealer. I'm really pleased with them now. I couldn't justify the £90 for a Snugpak one the zips where only 7 quid each and are ykk heavy duty ones.