A friction-held spool holder for larger 2KG and potentially 3KG spools. (I have not tested it with 3KG, but it should fit. Worst case scenario, stretch the horizontal pole a bit longer)
Two brackets hook under the rear of the frame, and the front-facing part provides bracing/support from the forward pull of the filament unspooling. Slot the two legs into the rectangular holes.
Horizontal bar comes in two pieces; you may want to glue them together as the fit is kind of loose. Once combined, simply let sit in the u-shaped top part of the two legs, as pictured.
10% infill was fine for a 2KG spool, but you might want to get a bit thicker for 3KG spools.
I've used this for a few years!