First congrats with Fret being broody again!I'll try to remember to take a picture.
You need a piece of this,
https://shop.deltarubber.co.uk/black-epdm-rubber-sheet.html
and a piece of space blanket (cheap is fine) and a roll of double sided tape.
Cut both over size, stick one to the other keeping the foil as crinkle free as possible and then cut to nest box floor size. Put usual bedding on top.
Second a question about the rubber. Are you sure it’s not made from recycled car tyres? There is a discussion going on about the toxics in it: https://www.backyardchickens.com/th...-water-30-mg-l-2000-times-epa-limits.1620309/
( @MaryJanet )
You can put anything under the nesting material that isolates. But I would prefer something more natural. I know a guy who claims a clump of grass complete with roots and sand is perfect.