The LRM turret seems a good idea, but the Medium Laser Turret not so much. The Laser Turret's BAR 7 armor with only 6 pts on the turret and 4 pts on a side mean that it only needs at most 3 Medium Laser hits or 2 Large Laser hits to be destroyed. Since its max range is limited by the Medium Laser, that means an opponent could sit at range 10 and get the Immobile target modifier to hit the trailer.
For the LRM Turret, possessing only 2 pts of armor on the sides and 3 pts on the Turret it will be popped by nearly anything, even before taking into account the BAR 7 armor limitation.
Recommendation:
Only use the Missile Turret, remove the SRM-4 and the SRM-4 ammo, replacing it with another LRM-5 and another ton of armor.
The extra ton of armor will be split between the Turret and whichever armor facing that will be painted "this side towards enemy". With two LRM-5 the Missile Trailer will only have 12 salvos of LRM fire, but it is more likely to be able to use that ammunition before it gets picked off and blows up. By adding half a ton more armor to the Turret and one side, this changes their protection to 14 for the Turret and 13 for the facing. With these values, an enemy will have to use something serious to destroy it even with the 'Immobile Target' modifier.
Best might be using it with LRM indirect fire. Park it behind a hill so it can't be seen, and this way an ally can call upon the LRM fire without worrying about their support being picked off. If there is no convenient hill, find out who has a Bulldozer on their vehicle and bother them.
Another option might be the
Mech Mortar/1. Compared to an LRM-5 the MM/1 is one-quarter the price, has the same ranges, same shots per ton, ammo is one-third the price, but the weapon only does ~half the damage. If you need a cheap base defense a few of these might be useful. Even better, if it blows up it is only 1 ton of ammo, aka 24 shots, at 2 pts of damage per, meaning 48 pts of ammo explosion. Compare that to an LRM-5's 24 shots * 5 missiles/shot, for 120 pts of damage. Even when it blows up the MM/1 is safer for your troops.