I would recommend actually buying a transformer, it's not expensive.
9v 2.1A is around 20VA, so anything higher than that would work.
Ex.
27VA 9v/18v
https://www.digikey.com/product-detail/en/hammond-manufacturing/266K18/HM2140-ND/235804425VA 9v/18v
https://www.digikey.com/product-detail/en/triad-magnetics/VPT18-1390/237-1323-ND/2090061Get the toroidal transformer, put it in a plastic project box, install a IEC C7 connector on the box and the cable with barrel jack connector or whatever your device needs on the other side and you're done.
If you want to modify one of the transformers, probably the easiest would be to get the 15v AC 1.5A one and unwind the secondary winding, measure the length and then cut a bit of the wire to match the 9v ... you'd get 9v 1.5A ... hope that your device doesn't actually need 2.1A and is happy with 1.5a
To increase current, you'd have to mess with the diameter of the primary and secondary wires, calculating magnetic flux and other crap.
With the 24v transformer, maybe you could unspool the 24v secondary winding and make 3 x 8 secondary windings and parallel them and hope that your device will be happy with 8v AC
Or maybe make 2 x 9v secondary windings and parallel them.