Bit of an odd one I know, I'd like to shorten my air mat, it's too long for my bivvy bag, 189cm and made of nylon, no internal padding (alpkit cloudbsse).
Is there any means of re sealing this form of material properly if cut?
Hard to do that properly I'd say. Rather than cutting, could you roll/fold the end to the right size then glue it in place? Much less likely to leak than cutting and re-sealing.
Fold it over when inflated to make a pillow as Ross suggests, use duct tape to hold in place, should deflate/ inflate OK if not taped too tight. Duct tape will come off if needed for other repairs on the hill.
This has been discussed on the various lightweight gear forums, such as backpackinglight. I recall various success reports, as the seam sealing method appears to be thermal; shorten, clear foam from new seal area, apply heat & pressure.
Could you put the bivy bag on top of the mat rather than put it inside? That would avoid the need of modifying and potentially damaging your mattress. It would leave plenty of space for your sleeping bag to loft too. Wild sky gear on eBay sell lightweight ground sheets for around £12 for protecting mats from the ground
In wet weather you end up in a puddle in the mat outside the bivi bag. Two solutions, make sure cambered ground, uncomfortable, or cut holes in camping mat, not good for inflatable mat.
It needs to be a really new g tex bag to withstand submersion.
