I use handwarmer pouches as a simple alternative to a dew strap from my refractor and guidescopes.
You just open the plastic outer bag and use a rubber band to hold it in position near the lens cell.
The best deal I have found is this:
That’s about £20 for 20 pairs, so 50p a pack.
The packs contain salt and iron filings, and when opened moisture in the air causes the iron to rust over about ten hours giving a gradual but worthwhile release of heat.
They are non-toxic, but beware – if your labrador east one the salt content makes it a very effective emetic…