Using the Remacri upscaler in Automatic1111:
Get the '4x_foolhardy_Remacri.pth' file linked in this post
Copy it to: \stable-diffusion-webui\models\ESRGAN
Restart WebUI
4x_foolhardy_Remacri is now available in the Extras tab and for the SD Upscale script
I didn't create this upscaler, I simply downloaded it from a random link on reddit and uploaded here as I couldn't find it anywhere else.