jaege. I have built 3 bottomless pits. I used the plexiglass and gila film for all three and they looked great. Here is a pic of my latest one using a glass mirror on the bottom and plexiglass with mirrored tint film on the top piece.
I liked the plexiglass for the very reasons stated. Weight and durability. It is a lot cheaper than buying a piece of one way mirror. They are so very expensive. I bought mine at Home Depot. A package of mirror tint is about $25 but I change my film each year, so it lasts me about 3 or 4 yrs.