This is a Spool support drum for Anet-A8 stock spool holder. This helps in reducing the strain on the Filament as well as the extruder (when it reaches the farther edges, as a result will reduce the layer-jitters that you can see on the edges of your print model sometimes).
Also it prevents the spool from being pulled towards the printer and colliding with it.
The dimensions are 100mmx50mm, inner hole diameter 9mm (designed for 8mm spool holder). The diameter is kept 50mm because, the various spools that I have used till today have hole diameter ranging from 52mm to 67mm.
After adding this, the spool doesn't get pulled anymore, so it does the job.
Ext_temp:210, Bed_temp:65, Print Speed:50mm/s