The pack frame makes use of your sleeping pad to support and cushion you from the contents of the pack. You can use just about any type of open or closed cell foam widely available at any outdoor shop or site. I would suggest sticking to an open cell foam with a thickness of no more than 2 cm because it offers the best weight to insulation ratio (compared to closed cell ones) although with some sacrifice to durability.
You can use the accordion-type Z-rest compact closed cell foam pad. They are commercially available in long (fourteen 5" sections) and 3/4 (ten 5" sections). Both have a thickness of 3/4" and a width of 20" so you may have to trim it to fit in the mesh backpad holders of the LAB. However, if you're a really tall person and you made the pack according to your body dimensions (i.e torso length values equal or greater than 23") then you need not do any trimming at all. But in most cases, one has to trim the width of the pad to fit the minimal confines of the backpad holders. If you get the long Z-rest then you can cut it into two pieces with one having 6 sections and the other 8 sections. An important thing here is to fold them accordion-style with equal number of segments per side before slipping them into the holders. Use the 6 section for your ultralight needs and the 8 section for more luxurious camp-outs. Remember though that the longer the pad (8 sections) the harder it is to fit or put inside the holders and the farther the pack's mass from your center of gravity (translation: less stable and less comfortable) because of the thickness of the folded pad. An 8 section Z-rest folded to fit the LAB pad holders would have a thickness of 3" - that's quite thick. I encourage using the 6 section pad with a folded thickness of 2 1/4" instead.
But if you can find an open cell foam pad with a thickness of about 5 mm and cut it to a 30" length (the width varies with the packs size), then go for it.
Mine measures 30" long x 14" wide x 5 mm thick and I folded it twice to have a sort of 3-section arrangement before slipping it into the holders. It weighs a mere 1.8 oz and can insulate my head, back up to my waist area from the cold, damp ground. The following illustration demonstrates the type of pad set-up I use for the pack's frame.