I think the pre-fab kits overpriced, too, but the original Interslice kit wasn't. I paid $10, postpaid to the UK from Texas, for Mr Goodman's kit, which had a pile of useful components, like sealing strips cut to the right and uniform width, foam patches of different thicknesses for mirror damping on different SLRs, and other sheets of different thicknesses that would handle trickier jobs like the Ricoh 500 series. The mirror foam had adhesive backing which could be manipulated easily into place by wetting it. At the time, it cost about the same as the mousemat I cannibalised. The bamboo stick didn't work that well, that was all. That kit was well worth the money, and I wish he'd restore it to his inventory. All the same, these jobs are still eminently possible with the assortment of craft store products, and that's what I use now. The warning about door hinges is a good and timely one.
At the risk of overloading the original poster with information, here's another useful description of light seal replacement, this time using wool :
Lots of fun for when winter nights begin drawing closer.