So, the amp measures 11x18x24", with a handle on top that I'll need to accomodate. My initial plan was to make columns of horizontal stripes, using the jeans/pants materials that you see cut into strips above, and then line it with the t-shirt material at the top and quilt the whole thing.
After all the previous discussion about using knits I've decided to forego the lining/quilting for this project and instead embellish the completed cover with embroidery. And in the process of sewing the strips together I decided I actually like the look of rows of vertical stripes better instead. I also realized that I didn't have enough material so ended up slicing up another pair of misfitting pants (the darker brown below) and will incorporate that into the rows as well. Below you see smaller blocks of stripes --- I'll combine them into different-sized rows, depending on the width needed.
I was planning to edge the bottom and the handle opening with the brown ribbon, but now that I've added in another brown fabric I'm thinking I might switch to a navy blue ribbon for edging instead.
Feeling very happy about the progress I made yesterday. I can see that I didn't cut all the strips on the same grain, something I want to pay more attention to in the future (I did start paying more attention to that towards the end). But this is not something that is ever likely to get washed so I'm not going to worry much about that right now. Looking forward to assembling everything over the next week and then doing some embroidery.