I have only found two such packages available for Linux, one is called VeroRoute and is rather intriguing but not perhaps suitable for the average user. The other is called DIY Layout Creator (DIYLC). Not authored by me, but since I find it useful I have created an unofficial deb package for it. However I think Flatpaks are definitely the way for the future and would like to see a Flatpak. Although I am able to create a simple one I am not able to do this for DIYLC as I am not sufficiently expert in flatpaks and have no knowledge of the language DIYLC is written in - Java.
DIYLC is a very professional and effective program which is undergoing regular development. I wonder whether there might be a Flatpak expert who would be willing to create a Flatpak for it?
Name: DIY Layout Creator
License: GPL version 3
Upstream has been contacted: Yes