If you type testcheats=1; into the ~ console, you can press F10 (F11?) to open the GUI editor, where you can make and save GUIs. It's disabled as of 1.12 for what I can recall (not much at all), so either use an old version, OR open in in MBG. I don't know if it'll work in MBG, but the key actions are controlled in /scripts/default.bind.cs, so if you tweak it you can get it to work for you. And just copy the dso that would compile into your game and you can open it there.