How could I create a sandbox based gamemode with different lua mods?

Well for various machinima reasons, I don’t really need this code for multiplayer, but more for a singelplayer machinima or just to add to my server and mess around with friends. I really just want to take the basic sandbox gamemode and add a few tweaks(which I can do by myself), but the main point is that I want to add things such as (ex.) “Points mod” or something like “Locked doors mod”. Could anyone tell me the correct way to create my own gamemode for this?