I don't know if this will help anyone else, but it seems to be the best option for my game.

Code
foreground = GetForegroundWindow();

		if(foreground!=hWnd)
		{
			beep();
			proc_mode = PROC_NOFREEZE;
			freeze_mode = 2;
			while(1)
			{
				foreground = GetForegroundWindow();
				if(foreground==hWnd)
				{
					freeze_mode = 0;
					proc_mode = PROC_GLOBAL;
					break;
				}
				wait(1);
			}
			
			wait(1);
			beep();
		}


Since my game does not need to run in the background, I just froze it whenever it was not the foreground window. It's based on Emri's code, but adjusted slightly. This is all in another while loop that I use for other things, like calculating fps and some other general purpose scripts.

See my games here: https://www.indiedb.com/company/michael-klaus-schmidt