So this might sound crazy, but here is what I did. I used thin black felt and glued it to areas of the Head Up Display Well. First off my interior is black with a near black dash. If yours is a different color you will have to get a felt that is close to the color of your dash.
Basically the glass that the digital display is behind is at the bottom of a four sided well. Some of the reflection is coming from the two inside vertical corners on the side of the well closest to the driver and along the top edge of the same side. Also the left and right top edges of the frame and the radiused edge that wraps down the vertical sides.
All around the top edge is about a 5/16" wide frame, like a picture frame and some of the glare is coming from that too. The frame also has a nice small groove around it so you have a clean line at which to end the felt. I covered the side walls and the wall closest to the driver and the picture frame with a thin black felt.
I used paper to create a template for cutting the felt to the shape needed. I had the paper a bit bigger than needed and pressed it in at the corners and along the top edges to create where to cut the paper to size. I then laid that on the felt and cut it to size.
Note: I actually created three separate pieces of paper and felt. One small piece for each inside corner and one for the side closest to the driver. The one closest to the driver not only covers the inside of the well but comes on to the top of the dash covering the frame along the drivers side and going back a ways on the left and right sides.
I used a very thin double sided tape that is used in picture framing, it is 3/4" wide with a peel off paper on the second glue side, and I completely covered the backside of the felt. I applied the little corners first, then the side closest to the driver overlapping the corners and and the frame along the drivers side and going a ways back on the side top edges.
Here is a link to the type of tape.
https://www.dickblick.com/products/atg-transfer-tapes-dispenser/ I sold photographs at art and wine fairs for years and still have a roll or two that come in handy for many things. I cannot give you a good place to purchase it but this link shows you the type of tape. The brown roll in the link. It is super thin. Do not use the thicker types of double sided tape as you will see it under the felt along the top edges on your dash.
Be careful to keep you fingers clean of the glue, but if you get some on the dash this tape will rub off with your finger if some gets on the dash.
I have included two photos of mine, one from the drivers view and one looking through the windshield.
This works real well but does not completely eliminate glare all of the time. Depending on how the sunlight is coming through the windshield, glare shows up in different locations and patterns. Some I am pretty sure are coming from the glass in front of the digital display which I don't think are possible to get rid of.
One way to determine where the glare is coming from is to have some small pieces of felt or black paper with you in the car. If you are driving and glare shows up and you can pull over with the glare still showing, you can use the felt or paper to cover different areas of the well and top edge of the dash to see what needs to be covered to stop the glare.
Hope this helps, I am happy with mine. The glare is gone most of the time.
I also got a roll of flat black stripping tape 1/4" wide and covered the thin chrome strip along the top front edge of the dash. There was always a reflection / glare line going across the top of the windshield that bothered me. This worked really well.
Hope this helps,
Tom