The description of the immovable object spell says (Explorer's Guide to Wildemount, p. 187):
You touch an object that weighs no more than 10 pounds and cause it to become magically fixed in place. You and the creatures you designate when you cast this spell can move the object normally. You can also set a password that, when spoken within 5 feet of the object, suppresses this spell for 1 minute.
If the object is fixed in the air, it can hold up to 4,000 pounds of weight. More weight causes the object to fall. Otherwise, a creature can use an action to make a Strength check against your spell save DC. On a success, the creature can move the object up to 10 feet.
It seems like I could cast immovable object on something like an umbrella and put it in front of me, and then nothing would be able to penetrate it, essentially giving me full cover. In order for it to be penetrated, it requires something to pierce it, or in other words, move the pieces of the umbrella apart. Am I reading this correctly?
Best Answer
Maybe half cover
You're hiding behind the umbrella and said umbrella is extraordinarily difficult to move through, so it could serve as cover. Whether the object's resistance to being moved from the spell confers it additional ability to withstand damage is a matter for DM adjudication.
Overall, this would meet the definition for half cover per the cover rules:
That said, there's nothing preventing your foe from just walking around the umbrella.