I really liked the idea of this thing: https://www.thingiverse.com/thing:2680818
I changed the desing a bit to better fit my demands. I also added a duct for the filament. This spool holder can hold spools with at least 60mm of inner diameter and a maximun width of 70mm. It will fit the right side of your Anycubic i3 Mega. As mentioned in the original thing-description the stock screws will not work with this print. I have aligned the holes to match those of the stock screw holes (with some extra height adjustment), but you will need screws that are 3mm longer than stock. I believe the stock screws are M3x5 and need to be M3x8.