This is what I did, but you may find better variations.
I wanted to use a resistant material, easy to find, easy to cut, and foldable. I used a milk brick.
Beside the piece I placed a card (credit card size) to give you a visual reference of its size.
I wanted it to fit in my wallet as any other card.
Fold it in half, do two vertical cuts, and 2 cuts with some inclination. The separation depends on your phone+cover.
Note that there are 3 folding lines.
Now you can place it on a table and use any of the two cuts to place your phone (vertical or inclined).
The phone always rests between 2 holding parts (actually 2+2).
Note: The phone on the photo is a bit wider than the phone I designed it for.
This is how it looks against the phone.
If you find better alternatives or have suggestions please leave a comment (no account needed).
There are a million things you can do yourself with just a bit of imagination, scissors, tape, etc.
Hope itĀ helps.
DIY: Do It Yourself.