I originally tried to use Alex100dre's cable guide for the original Anycubic i3 Mega, and discovered it doesn't fit on my later i3 Mega with Ultrabase.
I completely remodelled this from scratch to work for the later Anycubic i3 Mega with Ultrabase.
My version is slightly narrower and slightly less deep to allow it to fit. There's a larger notch in the back to clear the connector used on the later i3 Mega, and some additional support where the guide joins the base (because I accidentally snapped the cable guide from the base on one of my test prints).
Anycubic i3 Mega Ultrabase
Printed with the black PLA I got with the printer.
You will need supports on, to support the cable guide part, but they should snap off relatively easily.
You might find that the screw holes are a little tight. I found it easiest to screw the screws in to the base, before trying to fit it to the printer.