If they can engineer a revolving radar into a container then why not a revolving turret and if they can engineer it to pop up as you say then surely for it to just sit there is an even easier mod as half the job (and arguably the more technical) of "popping up" is negated. Stanflex and patria nemo amongst others use standerdised containers as modes of transport so the concept is not actually new or as mind boggling as you think and if it can handle a 120mm mortar turret then I'm pretty sure a 25mm bushmaster is still doable on the old engineerring scale
I’m not saying it can’t be done, simply that the juice isn’t worth the squeeze. Spending multiple millions to design and engineer a bespoke solution for a tiny number of systems is never going to be cost effective. It would be far cheaper, and far more useful, simply to continue to use the turrets on the vehicles.
Any weapon system can be overkill in any situation until you need to use it but luckily it also has a co-axial 7.62 if you a feeling frugal and want to keep detterrance to a minimum, and therein is the point, convincing the enemy not to attack somewhere it is, ie your base, which as you say also cannot move.
Overkill in this instance is not a good thing, as it detracts from the military principles of economy of force and main effort. Obviously, you want to expend the minimum resources you can on force protection. No one has ever won a war by defending their own bases, and the more effort expended on base defence is less effort expended on defeating the enemy. This concept would only have value if it was more economical than the alternatives, and that just isn’t the case.
For example, all the other containerised systems for base defence are remotely controlled. That is so they can be remoted into the CP/TOC, and one or two guys can monitor and control all the systems across the whole base. Obviously, this saves significant manpower. A containerised DELCO turret does not achieve this - you still need a two man crew for each system, with all the limitations below.
The training and manpower constraints of this system would be significant. Normal base defence is small arms, and maybe a HMG/GMG or two. This means more or less any soldier can man the towers, and everyone can rotate through tower piquets. Put a containerised DELCO turret there, and suddenly the only people that can man it are qualified LAV crewman. Therefore you are going to either have to qualify everyone on the DELCO turret (very resource intensive) or you are going to need dedicated crew that do nothing all day other than man the turrets. That is a significant manpower burden if you have multiple systems. If you already have LAVs in theatre this is less of an issue as you rotate the crewman through, but if you already had LAVs in theatre why would you need this system at all? Just have firing positions for the LAVs around the perimeter and you’re golden.
Which leads onto support requirements. Put a containerised LAV turret into theatre and you need most of the support you would need for a LAV proper. You’re going to need qualified gun plumbers to maintain the weapons, qualified tech elecs to maintain the electrical systems, qualified spanners to maintain the underlying system. You’ll need a supply of 25mm ammo, plus a space to store it. You’ll need a facility for bore sighting and zeroing. I’ve already mentioned the crew constraints. If you are going to go to that effort to support a few systems for base defence, you might as well just deploy LAVs anyway.
And finally, the entire system just wouldn’t actually be very good, with a lot of operational limitations. For example, you don’t have elevated sensors - you still only have the sighting systems on the top of the turret. Even a normal tower picquet would have sensors more elevated than that. The gun itself would have very little ability to depress, particularly if it was bolted to the top of a container. You therefore are going to have significant dead zones where you can’t apply fire anyway. As FOBs are generally built on elevated terrain, limited depression is not a good thing. The inability to move, at all, is also a significant limitation. Small arms, even HMGs, can be moved to cover primary, secondary and alternative arcs. Vehicles can be moved into different firing positions to achieve the same. A containerised system cannot, and therefore you are going to need a large number of systems to cover a useful portion of the perimeter.
Cost effectiveness is relevant as in we will already own these turrets, like we already own 20 "surplus" vehicles due to lack of use but unlike their vehicle mounted versions they are actually not much use without a base of some description, vehicle or other, so any removed turrets from any re-roled vehicles are literally wasted money.
Have a read of the sunk cost fallacy in business. Throwing good money after bad is not cost effectiveness.