Sorry I haven't replied until now about what I'm doing -- there are obviously a lot of ways to do this, including heating up various items, normal heating pads, and other heating elements. Here's what I do, but everyone has a unique situation:
I am also an avid gardener, so in February/March every year I am starting seeds in trays/flats in my garage. Most seeds actually germinate in 70 degree soil, so they make seed warming pads to put underneath them. I don't have a normal "human" heating pad, and any old electric blankets would be torn up (as mentioned), so I use the seed warming pads! So, I put a double layer of aluminum foil beneath the pad, put the pad on, then set the crate (with tray) on top. The foil is just to reflect the heat upwards and not down into the wood flooring (no carpet) that the crate sits on. There may have been a slight warping of the plastic tray, but nothing major, and it is mine, so I don't really care.
I normally place 1 to 3 towels/blankets on top of the tray for Bombay to lay on, and when combined with the crate covering, creates a nice warm den for him. Every time I open the front door, I'm very jealous and want to crawl in there and sleep too. I think that's about it -- I leave it on 24/7 since it doesn't consume major amounts of electricity. The pad itself is about 1 foot by 2 feet - give or take. I put it under one corner of the crate, so Bombay can decide whether he wants to be warm or not.
Hope it helps!