What Aimless said. Fold lengthwise & roll it up.

I also have an REI UL Cruise 65 pack (2nd gen.) and stow my ancient 12 year old Thermarest Lite mattress in the bottom of the pack, next to my WM Megalite down bag. I even have room for my TT Moment beside them when stuffed vertically.

Of course I have added REI aftermarket side pockets (no longer sold) on each side of my pack at 400 cu. in. each so my main pack is not overstuffed. I consider the side pockets very necessary for 1st aid kit, stove & related stuff like windscreen, etc., SteriPen & Katadyn ClO2 tabs, funnel, etc. Then there's the potty kit of TP, hand sanitizer in a ziploc and a snow stake/trowel.

Side pockets are essential on any pack for getting at "necessaries" fast.

BTW, my WM Megalite always goes in a lightweight Outdoor Products rool-top waterproof bag. Same for my clean clothes.

Eric