I've seen the instructions for shortening the Neo-Air and the standard Thermarest, but I haven't seen any for the Big Agnes inflatable pads. With a much lower price than the Neo-Air, I'm sure there are a lot of BA pads that would get more use from the folks here if they could be made to a custom length.
With a baffled pad, you can't just cut it off and seal the edge as easily as a pad without baffles. The material is the same type of heat seal-able fabric, but the baffles interrupt the edge when you cut the pad. I found a way to get around this.
Shown here is an Insulated Air Core pad. The shortest they are available is 60" in mummy style and 66" in rectangular. I'm shortening a 20" x 78" rectangular to 48".
1. Cut your pad. Add a couple inches to the finished length you want:
(Pretty pathetic insulation in there)
2. Trim the insulation back an inch or so:
3. Trim the baffles back. Carefully cut the baffle material as close to the outer material as possible. There should only be about 1/4" of bonded baffle material remaining with no loose edges.
4. Since the trimmed baffles don't have any adhesive you need to use something else. I found Stitch Witchery at my local fabric store. It's a heat seal-able tape used for joining two pieces of fabric. Cut a piece and insert it over the trimmed baffle and iron it down following the instructions on the package. Take care to make sure the edge material is laying flat, without creases.
5. After all of the baffles are sealed you can begin sealing the rest of the edge. I did each section between the baffles individually. Be careful not to melt the material. I got a couple places a little too hot and it got soft, but it didn't damage the material.
6. Trim the edge and round the corners to make it look a little neater.
I inflated my pad immediately and I'm happy to say it's held full pressure for over a week. I feel that the edge seal is as strong as the factory seal. Hopefully this will help out someone else that wants a custom length BA pad.
Disclaimer: I assume no responsibility if you try this and your pad fails. This is how I successfully did mine.
Edit: I forgot the important part - weight before was 30.9oz. and after is 20.5oz. A pretty substantial savings.