Sewing, Repairing and Reducing Waste

(just visualize a very rough, thick, quilt/rug to go on the floor)

I've sewn a bunch of old jean pieces together, mostly to learn how to sew jean material with a twin needle (only broke one so far) on my cheap sewing machine, but I didn't think the project through completely.

There are two problems.

1. It seems the quilt batting I can find around here is pretty thin (1cm) and pretty expensive: CHF18 (US$23) per meter, and it is only 150cm wide. So to double or triple it up to get some decent thickness, for an ~84cm × ~84cm top and my scrap project is starting to look a tad expensive (like near enough to half the cost of a sewing machine). Any thoughts on stuffing material (it's just a dog bed after all) - or should I just suck it up and buy the batting? I've looked at using a rug purchased from a low-end Ikea-type store, but they seemed too thin and too expensive - and it goes against the reducing waste idea, right? I thought maybe I could piece together a second layer of batting from the remnant of one purchased meter, to get a bed that's at least a couple of centimeters thick, but wasn't sure how much integrity a separate strip would have when just tacked in (see problem 2). Or I guess I could cut the cover down to under 75cm × 75cm; it's not that big of a dog. Any feedback?

2. I was planning on using an old shower curtain for the bottom under cover, with through tack stitches, instead of buttons, at the checkerboard corners: no problem there I think. But making a bag, to put the filler in, by sewing it inside out leaves one edge different than the others (as the last closing seam) when it's turned right-side out. How do quilts normally get closed on all sides?