For my college-level US history classes, I often bring in original items from various periods, and have several WWI era field gear items. I knew that the original gas masks are typically expensive, and the ones that are cheap are usually in BAD shape. So I set about to make my own. It can still give the general idea of an American-worn SBR without the cost, or the fear of damaging an original mask.
The mask part itself is stitched from heavy canvas, and I found some steampunk goggles for the lenses. The bag I made myself as well, although I still need to add the leather tab on the left side. The filter was the really tricky part. I didn't want to risk buying an original or similar one not knowing what all was in it or it possibly being unsafe for students to handle, but try as I might, that shaped metal can is hard to find. So, I sanded down a block of a 2x4, wrapped it in paracord, caulked the edges, and painted it with four coats of glossy yellow. I found a good downloadable template for the instructions and made sure to put grommets in it as well.
Obviously it wouldn't past muster in a reenactor's circle, but for a total investment of under $25, it'll definitely work for me!