Building something that would be bettr than a fastory filter is possible. If you want to build one similar to a gas mask check out
Improvised Gas Mask it can get you started.
Building a custom filter should not be much work if the original filter is a simple shape. It could mostly be built of lexan or some ABS add a gasket around the edges and size it as large as you can fit. If you wanted to it might be possible to open the bottom where the filter sits now and open it up for a larger filter (possibly deeper).