At rootstem we don't just do commission painting, we like to contribute to the hobby in general. We have a section on our website called The Bunker where we put a collection of Army lists that we have built, campaign info, and even custom missions.
We have recently just uploaded one Warhammer 40000 custom mission in the form of shuttle run.
The idea behind this mission is for the attacked to attempt to take a landing pad or a supply building away from a small defending force before the rest of the defenders can arrive to counter attack and take it back, all while the small defence using the supplies to heal troops and fire upon the enemy with them running out of ammo.
Our custom mission and rules can be found here. Give them a go and let us know what you think.