I've posted a free sewing pattern for a basic seat pack. You can print the pieces out on regular paper, and there are comprehensive instructions with photos. You can use canvas, vinyl, cordura, upholstery facric, tweed (I hear tweed is the thing these days), etc. It's a great recycling /up-cycling project -you can use old bags, packs, luggage, jackets, banners, handbags, used inner tubes, etc (although the instructions also have a list of where to buy new materials). It's entirely possible that you can make one without even needing to spend any actual my eyes at all, so if you need one more Xmas gift but you blew your wad on Black Friday you'll be all set.
The link is here: http://www.dillpicklegear.com/pickle...e-saddle-pack/
The final assembly is a bit awkward, but it's fairly straightforward. You don't need a lot of sewing experience if you have some patience. Hopefully the instructions are clear enough.
The pattern is free, but I only ask that if you use it you consider making a donation to Bikes NotBombs, League of American Bicyclists, or some other good cycling advocacy organization.