don't buy cheap
I have a fab shop and have several auto darkening hoods. Advice buy the Hobart elite from ebay. About $200. I have a harbor freight that I bought to arc weld and MIG weld with because the cover glasses are easily replaced. The Hobart cover is a bear to replace. I use the Hobart to TIG weld with and it's cover stays fairly clear for a long time. The catch is the harbor freight hoods "clear state" is darker than the Hobart's, that is it is clear but like looking through sunglasses on the shade where as the Hobart is lighter like you turned on room lights with sunglasses. Both have adjustable shades and are OK when welding.
Also if your over 40 buy a magnifier lens to go inside the hood, your welds will be better and you life may depend on how good your weld is sometime.
You can RTV them inside the hood and your good to go.