Yesterday evening, I walked into REI to browse. It wasn't my intent to actually purchase anything, but I do still have a few bucks left on a gift card, and I do need a few things. Before long, I found myself drawn into a review and discussion with several workers there that wasn't exactly what I came in for, but which was very useful nonetheless.
I've been thinking of replacing the bag with a slightly bigger one, but I won't find anything at a price anywhere near what I paid for that one; not without taking on a ton of bulk and weight. I looked at liners, and REI had some pretty solid looking ones, although they cost twice as much as my bag did. I still like them, but I'm not really looking to spend that kind of money to correct what is mostly a small problem.
I also saw tiny and inexpensive space blankets, and I thought both in terms of size/weight and price, that looked about right. I asked one of the guys there what his experience with them was. Naturally, he recommended the much more expensive liners, but he did make a few good points; the space blankets are very crinkly and loud, and they don't breathe at all. On reflection, I have vague memories of one of the kids I was a Boy Scout with bringing one of them instead of a sleeping bag one over-nighter, and him having a bad experience being cold and uncomfortable.
I still think the idea has some merit, but I'm no longer seriously considering using a space blanket as a sleeping bag liner. Oh, well.
Luckily for me, however, I already have more options. I, in fact, own three blue foam pads as well as a Coleman self-inflating pad. The latter is probably bulkier and heavier than I'm interested in, but it wouldn't be hard for me to roll up two foam pads together, bundle them fairly tightly and stick them together on the bottom of my pack, where the (minimal) extra bulk won't actually cause any additional problem for me to carry. Because the things are essentially weightless, carrying two of them is more a problem of bulk than of weight, but again; the way I can pack them will make that a moot point too.
So, I'm going to make a slighter tweak to my sleeping system than I had thought--I'll merely bring an additional pad of those I already own, and wear an additional skin-tight baselayer underneath what I wore last time. This should, I believe, do the trick. If, for some reason, I'm still feeling a little colder than I'd like, then I can investigate liners. I might do that via much cheaper fleece liners that I've seen in the sporting goods section of stores like Meijer or Wal-Mart, because it's much cheaper, but it would be bulker, no doubt. I do like the liners I saw at REI, but they're at least $60 a piece.