NOTE: If you like this thing check out my other complementary thing:
https://www.thingiverse.com/thing:2818006 (narrow version)
https://www.thingiverse.com/thing:2850749 (revised quick change)
http://www.thingiverse.com/thing:2067525 (quick change)
UPDATE 1/13/17: I have included 2 types of cross beams based upon some feedback that I've received. This is specifically to offer more stability to Version 2, now called Version 2.1. Please see the picture and STLs for specifics. I personally use 4 nuts and have no stability issues but I'm currently using version 1.
Update 4/7/17: I have split version 2 of the spool holder into 3 different parts as requested by WayneNero.
I wanted something easy to insert and remove my spool. Although I've been using the spool holder that came with my Anet printer up to this point, at times I've debated going to a top mounted spooler so that I can free up some space on my work table and ensure the filament feeds with little resistance.
I came up with two versions of my customized spool holder. I'm currently in the process of evaluating them. The first version is straight up much like ub0baa's make. His work served as the inspiration for both of my designs. This design may be ideal for the Anet A8 printer since balancing the spool on center puts less stress on the acrylic frame. For this design, you should use 30% infill and supports when printing.
In version two, the spool holder leans backward. For the Anet A6, which has acrylic cross sectional supports in the back, this is ideal as it will stabilize the frame toward the back end. I personally prefer this version, so as to not have the spool holder swaying back and forth if it were balanced on the top like my first version.
Version 2 comes in four parts. Two of the parts connect as shown in the picture for each side. I did this to minimize the amount of support material required. You could glue these parts together or just pressure fit them. I found that once mounted on the printer, the parts held together well. The use of supports is optional but use them if your printer doesn't handle angles well. I did use supports and contended with some minor cleanup afterwards.
The quick release clips, you'll need two of them, come from Rowkii's spool holder which I printed out and use too. They make securing the spool rod to the holder a cinch. I personally put them on the outside right up against both sides. You can use nuts just as easily too.
For stability, you should place your spool rod in the lowest rung possible.