How to map AltGR+key to a Unicode symbol

In Linux Mint, I am used to using AltGR to generate symbols, e.g. AltGR+4 produces € and AltGR+shift+2 produces ⅛.

I often want to use an italic curly L (Unicode 2113 ℓ), and would like to map that to AltGR+L. This is much easier to remember, and to type than “2113+Alt+X”. Also, it should work consistently between different applications.

I have tried the Mint Keyboard app and Input Remapper (https://github.com/sezanzeb/input-remapper) without success.

Any help would be appreciated.