Sphagnum moss works fantastic. Also bedding will help with increasing humidity. You can use cypress mulch, coco husk, coco fibre, or do a combination. If you’re on a bit of a budget I’ve used a damp towel and left it inside the enclosure and it works fantastic. I made mine drape over a couple of sticks to kind of make it like another hide, but however you would like to use it you can. Make sure you replace/wash the towel to prevent bacteria growth if you decide to do this method. Once a week should suffice.

If a heat mat doesn’t work for you then installing a overhead lamp could work. If your PVC enclosure is made of wood, you could cut a circle at the top for the lamp. Otherwise set you’re temps a little higher for your mat and read the temperature from the inside until you reach 85-92 from the inside.