I cant recommend a mat but I did experiment using various cloth and other materials on a Corsa (which was much worse for glare and reflections than my mk 4 Jazz) . So can maybe save you from wasting time and money.
Fortunately I only used things I had to hand and didnt spend money buying likely looking black/dark materials from fabric shops etc . I found most materials, however dense and matt they may appear ,still reflect a surprising amount of glare. Especially those made from man made fibres,as most are. . Someone recommended felt, but the sample I already had at home still reflected light. I considered using some black non slip matting.The type thats a sort of bobbly rubber open net. But it still looked reflective and I was worried it might melt in extreme heat.
The most successful was, as Marcus says, a bath towel. I tried one for a while, but without tailoring it to fit. It looked scruffy,moved about , and gathered dust. .
There may be better materials such as a special non reflective cloth photographers use in studio work.
You can buy custom made dash mats for some cars (inc the mk4) which I think are a sort of carpet type material. But many black carpets (such as car floor mats) are still reflective.
But whatever you use I would still have concerns about whether it will fit, remain in place ,or impede air vents ,sensors ,deployment of dashtop airbags, etc.
In the end I found the best solution for me was polarised sunglasses in bright daylight,(also available as prescription lenses if required) . And whatever you normally drive with (if anything) when its cloudy or at night. But I am fortunate that I am borderline for needing any prescription at all, and can quickly remove sun glasses and continue to drive safely and legally ,when going into tunnels or sudden shade etc.
