Do what I do:
Go to Walmart to the school supplies section. Buy the 3 panel display boards that are 36" x 48" for $1.99. Get as many as you need. Go over to Ebay or Amazon and order emergency space blankets. They measure 54" x 84" and come in various numbers. (One blanket will do two of the 36" x 48" boards.) I bought one that had 5 in it for 4 dollars. Use some spray adhesive to secure the mylar blankets to the boards.
Yahtzee! You now have solid mylar reflector panels 4 feel long, 3 feet high with adjustable panels for the grand total of $2.50 each at most. You can hang them, set them on the floor like walls to reflect for side lighting (which is what I do as I don't use a tent), anything you want. Dirt cheap.
And man, it's like you turned on another HPS when you get them set up.