I found this gas mask it looks exactly like the recommend type it's like an scba mask but you can buy it on it's own without the whole thing, if it's the right one why aren't more people just buying it directly?
I asked that very same question when I first joined Sasu, my whole method have actually been worked around this very concept.
Initial problems is that you can't directly connect a constant flow setup to an airtight system efficiently. To be able to breathe in such a setup it would require a really high flow to sufficiently supply nitrogen during that small window when taking a breath. When not breathing that high flow will build pressure around the mask and leak out around the edges of the mask or another weak point.
How eebd hoods work around this problem I'm not sure, but it has to do that the mask not being as airtight and the use of a neck liner.
My solution to the problem is using a secondary reservoir tank which is filled with nitrogen at atmospheric pressure. The mask is then connected to this reservoir tank and thus I have worked around the problem of not having enough nitrogen at each breath and at a lower flow rate. An overpressure valve is also fitted to relief the pressure inside the reservoir instead of the mask.
There's no off the shelf parts to construct such a reservoir tank and require some DIY work, mainly drilling and ensuring an airtight setup. Hoses from the mask are available in some mask, many use 40mm nato standard even on civilian products.
It isn't really that hard to construct such a setup, but suicidal people aren't famous for building things and many are in a hurry.