Customizable on-screen keyboard for games

[VortNs]

Hello.
During my attempts to play DOS games using emulators like DosBox or Bochs with Termux:X11, I discovered major problem: none of virtual keyboards I tried are suitable for this task.
For example, Jazz Jackrabbit game is controlled by keys Alt, Space, Left and Right.
Gboard does not even have these keys.
Arrow keys on Termux:X11 keyboard can't move character for long distances, Alt key is toggled and not released automatically.
Florence keyboard allows character to move with arrow keys, but fails completely to send Alt key presses.
Android app Magic DosBox Lite allows user to create custom overlay keys in arbitrary positions on screen, but it is close sourced.
Are there open source solutions to this problem, compatible with Termux? I saw direct touch option in Termux:X11 option, probably it can be used in combination with some separate virtual keyboard software.
Other possibility is to integrate such feature into Termux:X11, in a way similar to how mouse button overlay is implemented at the moment.