I've tried many "cheaters". I've found Optivisor by far the best quality.
Eg. http://www.loupe-magnifier.com/optiv...ens-plates.htm
They're made to fit the Optivisor headband but I've had no problem fitting them to my helmets. On one it's held in place with hot-melt glue dabs. On another, I cut a rectangular hole in a piece of clear acrylic that slots in place.
You need to take into account how you'll clean both sides. More importantly, they each have a specific focal distance. I use an LP-5 in my TIG helmet because the distance I hold my head away is just right for that lens. On MIG and Stick I use an LP-4 with a slightly longer working distance because for those 2 processes, the distance I hold my head is spot on for the LP-4 to be in focus. You've really got to match the lens to your comfortable working distance otherwise the arc-puddle business end is not in focus. I had to get an LP-3, LP-4, and LP-5 and experiment. I find I have to be aware when welding that I must maintain the correct working distance, or at least to be conscious that if the business end is slightly out of focus, it's because my head's not kept pace with the action.
Martin