The core of the new eSUN cardboard spools fits almost perfectly to the Bambulab reusable spools as it is just a bit narrower than the original Bambu spool core. This model fills the gap to avoid filament biting and any subsequent problems.
Printed in PETG for eSUN PETG spools (it may also fit other types of filament, I haven't checked yet them all).
UPD: The PETG adapter turned out to be too thick for PLA+ rolls, so I made another version for that type of filament. Use the correct one for your spool. PETG filler is 3mm thick, PLA+ is 2mm thick. Be aware that the spools are tight and tolerances are minimal. If you can't assemble the spool, you've done something wrong - the dowel didn't go into place, the filler wasn't pushed all the way in, or there's some cardboard or glue left on the core.
It should be mounted on the lower half of the Bambu spool which does not have the dowel.

Then tape the eSUN spool to prevent accidental unwinding, peel off one side of the cardboard spool,

use a knife to clean a core, mount it on the corresponding half of the Bambu spool (the core has a notch to be used with the dowel).

Then peel off the other side of the cardboard spool,

clean up that side of the core,

and snap the second half of the Bambu spool into place. Carefully remove the tape to not allow the spool to unwind and possibly tangle itself.
Ready to use in AMS.

The author marked this model as their own original creation.