See: github.com/40163650/FSRForReShade for more details.
It's essentially a conversion of goingdigital's shadertoy shader from GLSL to HLSL.
Feel free to tweak the rendersize variable to get it looking how you want.
Pretty major caveat: Your game still runs at full resolution. The shader then downsamples it to 1080p (kind of) then upscales it back to your screen's resolution.