This is how you can create your own pixel-exact custom background:
1. Start the game, enable "Take screenshot before filtering" in the Output-tab and then click "Save screenshot":
2. You now have a screenshot including all of the border area:
3. Only paint in the drawn area, then the background will fit:
The correct color palette index for the option genlock_effects I found out by trial and error. For Turrican it is 5, but for many other games it is 0.