A. Where possible it is best to avoid attaching anything to the outside of a rucksack. Items attached to the outside are more likely to get wet, damaged or lost. You must also be careful not to overbalance your bag as this will cause unnecessary back strain. It is possible to buy compression sacks which will squeeze bulky items down to a smaller size to make extra space in your rucksack.
If however your sleeping mat cannot fit inside your rucksack make sure you pack it in a waterproof bag before attaching to the outside of your pack. Ideally you should strap the mat vertically to the pack as this will minimize any damage from undergrowth or the ground, should you put your bag down. If you do not have straps available at the bottom of your pack for attaching the mat a bungee cord is useful.