I couldn't find one that fit my needs, so here is my design. It has an inside diameter of 70 mm, so small coils of filament fit easily. It has an outside diameter of 180 mm and a width of 20, 30, 40, or 60 mm so it fits normal filament spool holders or saves space as necessary, and the outer edge is fixed so edge-riding holders will work as well. The "Screw" end is compatible with any width "Base", so you can get away with only printing one if you wish.

Printed in two parts, the "Base" has one flange and the entire exterior hub. The "Screw" is the other flange and threads into the base. It is designed so that the base can hold the filament entirely, and the screw doesn't need any clearances to be attached. Each flange also contains 1.75 mm and 3.00 mm keeper holes.

Most edges are filleted, so the print time is a bit long, but it makes for a more friendly print and a smoother roll on edge-rolling spool holders.

Outer Rim Diameter: 180 mm (≈7⅛″) Outer Hub Diameter: 70 mm (≈2¾″) Inner Hub Diameter: 60 mm (≈2¼″) Inner Hub Width: 60 mm (≈2¼″)

The spool holder shown in the first photo is MartinMajewski's awesome "Prusa i3 MK2(s) & MK3 Spool Holder - Frame Mount - Top Loader"

Print instructions

No rafts, no supports, no brims. Just orient the STLs flange-down and print away. There should be zero overhangs. the threads are a generous 2 mm pitch with a 6g4g fit and should print and mate easily on any printer. I used 10% infill because of the small inner area, but you could probably get away with 0%.