If it JUST provides an FRP, perhaps it could be the ability of an unit, rather than a building or structure? The USF major does this, perhaps a new OST officer squad could be designed with this as an ability? (Or give it to the Artillery Officer, even)
Or perhaps there's some sort of unused Funkerwagen model that could get it. Unfortunately OKW already have the 223, and that has a different ability, or you could otherwise use that.
Actually now you mention it I had a similar idea before, giving the FRP ability to an Sd. Kfz. 260/261. This is basicly a turretless 223 (perhaps replace pintle mg with panzer commander), so it would be possible to make with the modding tools. My idea was that this vehicle would have dual purpose: it would have a very long sight range (70~80), so, if it is indeed put into Festung Armor doctrine, then you can either use it to spot for hulled down panthers (they have longer range than elephant actually) or deploy somewhere safer for FRP.