I see. I think this should be possible with the widget that KDE comes with.

You could also build your own very simple manager. There is a Wayland commandline tool to save or load something in clipboard: https://github.com/bugaevc/wl-clipboard It should be in the repositories of most distros I think. It comes with a copy and a past command. Using file redirection, you could write and read to files.

Following commands: wl-paste > ~/.cache/copybox/1 in example would save the current clipboard content into a file named 1 in the copybox (you can have any directory name, I just chose this as an example) directory under ~/.cache. You would need to make sure that the directory exist, so creating it first. wl-paste > ~/.cache/copybox/2 would save the content to a file named 2. And to read back a file into clipboard use wl-copy < ~/.cache/copybox/2

In your system you need to setup shortcuts for each of these actions to read or write the clipboard. Don't forget to create the directory you chose (in this example ~/.cache/copybox). What shortcuts you choose is up to you. Alternatively you could use a menu builder tool such as wofi, a tool that is similar to rofi. But that requires additional understandings and scripting. If you don't find any solution, then these could be your steps to create your own. Or maybe someone else does it.