"Another thing - if you just put foam inside a sleeve, it'll rotate around with use. Need to at least put a few stitches through fabric and foam to keep it in position. I've done hand stitches when my machine wouldn't get through it."
Leave it to Jerry to mention the most sensible method for the typical MYOG builder.
Questions: How much weight do you expect this pack to carry; and how long are your planned trips? The reason I ask, of course, is that most packs are **way** overbuilt. If you're doing MYOG, then the entire rationale, besides saving a little $, is to build something exactly to your requirements. That means not only measurements, but the strength & durability of the material(s) used.
I highly doubt that you need either your base materials and/or grosgrain. They both add weight, but more importantly, they're a freaking hassle as you're now finding out. By far and away, the simplest method is to take a common piece of fabric and sew a basic seam to form a barrel. (You might be able to pull off a flat felled if the sleeve is wide & short enough.)
Flip it right-side out and stuff your pad inside (ie the sock method), rotate the sleeve around so that the seam runs down middle of the pad, then run a stitch across the top/bottom (either by machine or hand) to prevent the pad from shifting and you're done. Attach your belt (I use 1/2" grosgrain in lieu of belt) to the top/bottom (or run it the entire length) and you're done.
In my case, my bear can pack is made from 1.9, so I just use another piece of 1.9 to form my straps. I believe straps should be wide, rather than thick, so my shoulder straps are 3" wide, while my hip pads are 5" wide. At this width, it's a piece of cake to roll & stuff in the padding.
Oh, and one last thing, I use neoprene exclusively for my pads. It's easy to find @ any general hardware store, typically sold in 1/4", 3/8" & 1/2" thicknesses by the square 6" for around $1. That means all your padding requirements will run under $10. Cheaper than cutting up a cheap blue pad, and way more comfortable to boot.