We have used that foam underpad that looks like a waffle pattern. It is made of foam "string" laid out in a criss-cross pattern, so it is thin, light-weight, and won't make the carpet look any "taller".
But it holds most carpets in place pretty well. Cheap, too, IIRC
I never thought to use that string foam stuff under rugs to keep them from sliding. Good idea. I use it to line my drawers and shelves. A good place to buy it is at Joann Fabrics. They sell it by the yard and it is about 54" wide. Can't remember the price/yd. but the savings is significant when you use one of their 40% off coupons. I think you can print a "40% off any one item" from their website or pick up a flyer in the store. Maybe you could even use a spray fabric adhesive to stick it to the back of the rug.
You can also find a rubber type of product made especially for rugs that you paint on the backside of carpeting. Since I sometimes do my own punch needle rugs I use that on the edges to make it stay put and lie flat.
I bought mine at a fabric store.
Larry and Lou
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