That 3m 77 spray glue will not come off the walls,and yes repaint and spackle
the walls.You can get 1/8" hardbroad {masonight} and glue the mylar on it.
You can get it in 4'x8' sheets at homedepot or lowes or any lumber yard.
Then use double face tape and stick it to the walls.Much easyer to get off the walls and put it some place else too.
Peace