Space Crusade AI Sentry Turrets

Overview: The Space Crusade ship has an AI defence system which attacks everybody. The Alien player gets to place the AI gun turrets to their own advantage. In return, the Booby Trap, Auto Defense cards are removed from the deck, and the Bolt Pistols card give you an extra red die instead of a white one. The turret acts like a stationary miniature until it is destroyed, or revealed to be a dummy. It shoots at moving miniatures from both sides.

This gives less randomness, fewer Marine Hand to Hand losses, and 

Optionally, each player may write down the number of gun turrets the alien player should receive. The player with the lowest number gets to play the alien. This means the Alien player has no advantage over the marines.

So, this improves Marine Hand-to-Hand capability, balances play, and adds strategy to the game.

 

 3d print Battletech Turret by Tokugawa

If you search for "turret" on thingiverse, you will come up with things to print similar to the above. Or you could get something from EBay, stick 2 cocktail sticks on a bottlecap, or use a token.
For 3d printing, a simple design would be best, since it is for a boardgame. You could make a cube with a cylinder sticking out of it on TinkerCad in a couple of minutes:

"Simple Gun Turret" on TinkerCad

Very Maginot line, you can find it on TinkerCad. So, this can be a proxy Tarantula turret for the marines (there is a better proxy Space Crusade Tarantula, Marine Squad, Chaos Squad, and Dreadnought out there too). or a static defence turret for the marines.

Rules:

1. The Alien player gets to place 2 turrets per 12x12 board section (1 board has no turrets if there is only one marine player) when a marine moves onto the board for the first time. The turrets may not be within the LOS of the Marine. Roll a red die when a Marine first sees the turret, 0 = dummy turret, 1 = attacks like a bolter with 2 white dice, 2 = attacks with 3 white dice, 3 = attacks with 4 white dice. Each real turret is worth 4 points.
2. If the first turret on a board section is dummy, the other is real.
3. The turret has an armour value of 2, and can be attacked as if it were a Space Marine.  It is removed from the board if it is revealed as a dummy turret, or when destroyed.
4. The turret fires during the Marine players' turn the first time a marine moves into a square within the turret's LOS. The turret fires during the Alien players' turn the first time an alien miniature (including Genestealers) enters a square within the turret's LOS. The turret may fire 1 shot in the Alien turn, and one shot in the Marine turn. 
5. The gun turret is in all respects an immobilized miniature. Blind grenades disable turrets for the marine turn and for the following Alien turn.
6. The Booby Trap, Gretchkin bomber, and Auto Defense cards are removed from the deck, and the Bolt Pistols card gives the marines an extra red die instead of a white one.

Instead of rule 6, you could give the marines overwatch for marines who didn't fire. I suppose that might kill 10 aliens

Comment:
The Alien player has new strategies, such as placing 2 turrets in the same room as the Dreadnought, so that is more interesting. There is less random card death for the marines, and The Blood Raven Blades and Bolt pistol strategy is now reasonable.
The Alien get on average 4 extra turrets, which is a bit like have 4 Chaos Marines that don't move.
The removed cards save 1.5 marines. Auto turret 2x1/6 = 33%, Booby Trap 58%, Bomber 58%. The Bolt pistol upgrade is probably worth about 2 marines. So that is probably a slight bonus to the Marines, with less randomness, and less effective suicide gretchkin.
If you want to tweak the balance, you can add or remove event cards or add more turrets.

Also, you can use the turret as a Tarantula turret using the Dreadnought rules.

Tarantula, tracked Tarantula, and the Turret model.

When a Genestealer appears, roll a red die. If it is 0, the Genestealer will be placed beside any Alien miniature, it will then chase and attack the nearest Alien miniature for the rest of the game.