#cold pack

The outside is a microfiber mitten washcloth, like these:

Doesn't have to be those ones, they're just a random example. A soft, small, cheap bag, that comes in a multipack.

Then, the cold part. Reusable plastic ice cubes (YES specifically the plastic reusable ones, so the washcloth doesn't get wet), like this:

Again, just a random example. Mine are spheres, picked them up as an Aldi Specialbuy. But it means when your cold pack gets hot, you can just swap out the ice cubes within.

Lastly, a bag clip.

Could be that shape, or any other shape that holds a bag closed so ice cubes don't fall out. If you're good with your hands, you could sew velcro in or something, dealer's choice.

The clips mainly come in multipacks, and lots of homes have them for food bags, so you might already have one- I did!

 Rice. 

Pros:

Affordable. Rice is easily available which makes rice packs cheaper to make and cheaper to buy. 

Takes essential oils easily if you want a scented heat pack. 

Can be heated or chilled. 

Smooth feeling, the small grains mean it is not as bumpy or uncomfortable to lie on. 

Cons: 

Fiddly. To prevent scorching you need to microwave with a small amount of water, which is not the case for most of the others. 

Easy to scorch. 

Doesn’t hold heat as long as some of the other options. 

Shorter life span than some other options, rice breaks down and will give off a strange smell and stop holdingheat as well with repeated use. 

Wheat. 

Pros: 

Affordable. Again, wheat is reasonably easy to get hold of so wheat packs are fairly affordable. 

Takes essential oils pretty easily, not as well as rice, but still easier thansome of the other items. 

Can be heated or chilled. 

Slightly larger grained than rice, but still pretty small and smooth, so could be more comfortable. 

Cons: 

Confusing. Some people recommend microwaving with water, to help hold the heat longer, but this advice varies depending on the humidity where you are. 

Can feel slightly damp after microwaving. 

The time it holds heat for varies. 

Shorter life span than some other options. 

Feed Corn.

Pros: 

Holds heat well. 

Longer life span than rice or wheat. 

Don’t need to microwave with water. 

Can be heated or chilled. 

Cons: 

Doesn’t easily take essential oils. 

Gives off strong popcorn smell. 

Feed corn can be difficult to get, so can be more expensive than rice or wheat. 

Cherry Pits.

Pros: 

Holds heat well. 

Gives off pleasant cherry smell.

Can be heated or chilled. 

Doesn’t need to be microwaved with water. 

Long life span.

Lighter weight than rice or wheat. 

Cons: 

Can be more expensive, but as they don’t have many other uses, can be cheaper than feed corn. 

Cherry pits are larger than any of the other options, so it could be uncomfortable. 

Doesn’t take essential oils well. 

Personally, I decided the Cherry Pits were the best options, partly due to their heat retention, partly due to their long life span. Rice is a good budget option.