Looked at other filament holders on Thingiverse and found most of them to be mounted on the side of the frame or off center at the top. None of the options at the time appeared to work for me so I decided to create my own. I wanted the least amount of drag on the motor and the weight of the filament to be balanced directly above the frame as it is acrylic. I would expect that over time that it might warp or shift if mounted off center with the weight being unevenly distributed.

Just note the inside width of the filament holder that rests directly on frame was designed with a slightly larger opening intentionally. I cut an inserted a small piece of foam to prevent any vibration that might occur with plastic resting directly on plastic. The foam dampens the vibration that might be transferred when spool is rotating.

I've also attached two different filament holder inserts as the inner diameter was not the same between them. One insert is for filament from "justpla" with an inner diameter of around 31mm and the other for filament from "Foldger Tech" which has an inner diameter of near 57mm.

The filament holder tabs file is an addition to the filament holder. This will help stabilize the holder and prevent prevent any tilting from left to right. The rod that runs through the filament holder is just a wooden dowel I found in my garage which has a diameter of 21mm. I chose this rod just for its convenience. You might be also be able to find something laying around that works for you.