How to make a heat pack for your pet rat

We have two small friends who have recently joined us in the bungalow: Kit and Pippa. They are 8 week old black hooded rats. Unfortunately rats have had a bit of a poor reputation throughout history, mainly from being blamed for the plague - which was actually spread by fleas - but they make fantastic pets. They're exceptionally clean creatures, they don't smell (unlike their cousins, the mouse) and they are exceptionally smart. I've had pet guinea pigs in the past, but they're daft creatures - cute, but daft. I like a pet that can think independently but still hang out with you and be sociable... and are cheap to maintain. A rat totally fits the bill.

Our little critters need to keep warm because they're so small and because it's so cold here in Melbourne right now. So Saturday afternoon I made them a heat pack (with assistance) to keep them warm at night.

To make your own heat pack for your small pets:
I made my heat pack with a double layer of fabric in case they decided to have a nibble. I made mine out of a light canvas, but you could use cotton drill, corduroy or any other similar fabrics.

You will need:

• 2 15 x 25 cm rectangles of fabric
• 2 12 x 22 cm rectangles of fabric.
• Lentils, ~300 g. You could also use rice or beans, but I'd not recommend wheat as they will be more inclined to eat that.

Make the inner pack: Sew around all four edges of the 12 x 22 cm rectangles, leaving a 5 cm gap. Clip the corners and turn out the rectangle. Fill with lentils. Stitch down the edge.
Make the outer cover: Sew around 3 edges of the 15 x 25 cm rectangles. Insert the inner pack. Fold in the edge of the outer cover and top stitch down to close. Top stitch around the remaining 3 edges.

To heat:
Microwave for 1.30 mins on 80% (800W microwave).

I had some assistance with this one

Perhaps I should make one for myself?