Sometimes hiking mats just come in the wrong length or width. For example, I would like a wider pad but they only come very much longer. Is it possible to cut a bit off them and reseal them? Yes it is. Here are some links on how to do just that:
How to Cut and Reseal a Neoair – On The Trail – Episode #1: https://www.youtube.com/watch?v=kumSl-cbLlY
Shorten Neoair: https://backpackinglight.com/forums/topic/73403/#625778
How to shorten a full length self-inflatable mattress: https://www.youtube.com/watch?v=AoRTAeKcA0w
Resizing a Therm-a-rest Evolite Sleeping Pad: https://hikelighter.com/2016/08/16/resizing-a-therm-a-rest-evolite-sleeping-pad/
Resizing the Massdrop x Klymit Static V Ultralight Sleeping Pad: https://hikelighter.com/2016/08/09/resizing-the-massdrop-x-klymit-static-v-ultralight-sleeping-pad/
People have slept thousands of nights on similarly modified pads.
Below are two mats I would consider cutting down to produce a mat which has more width comfort:
Thermarest XLite Large: 25” (63 cm) by 77” (196cm) by 16 oz (460 grams) R 3.2 If I cut this down to the same dimensions as my XLite Womens it would weigh 394 grams. Only 54 grams for that much increase in comfort! It would then make a superlative hammock pad for me which would not require any side insulation such as this: http://www.theultralighthiker.com/hammock-pad-extender/
Thermarest Neo Air All Seasons Large 25” (63 cm) by 77” (196cm) by 25 oz (710 grams) R 4.9 2 If I cut this down to the same dimensions as my XLite Womens it would weigh 608 grams.
I could cut an unnecessary 6” off Della’s XLite Women’s saving her 10% of its weight (or 34 grams)!
You might also want to trim a mat to make it more mummy shaped – and to save weight.
Other modifications: Erin McKittrick and her husband Hig during their ‘ A Long Trek Home’ (http://www.groundtruthtrekking.org/Book/) cut down their Thermarest self inflators by cutting a hole (roughly) in the middle of them through which they could push their head. They then passed a string/belt around themselves and the mattress creating a makeshift life jacket!