IF the Air suspension seems workable for the long term, you can also just put in a fixed rear spring pair on the back - just use a length or height and stiffness you can work with.
IF you can - read this thread -
https://www.fordfiesta.org/threads/...be-between-st-and-other-trims.7396/post-16194
Found a set of springs from MOOG that fit the outline bottom seat and works for holding the upper seat cushion - but lifts the vehicle up by about 1-1/2" to 2" rise - for me this works for the area I live in; being in Michigan and subjected to winters fury and lake effect snows that can add a considerable amounts of accumulation and provides its own level of challenges. So take the concepts for what they are worth.
That rise may not work for you - you may encounter a noise from the rear shocks reaching full extension before the car is finally at the peak of the rise of bounce during some driving conditions. Can damage those shocks, for it also requires a lift spacer to be applied to the shocks to the rear to help reduce the extension rise top-out bang heard during some rough driving conditions - I used Amazon to find a spacer kit to help with the shocks but the company that did this type of conversion kit was in a country that is now a part of Russia and so no longer does provide business for the world market. You can make spacers yourself using rubber sheeting (1/4" - in plumbers supply) and cut and auger to fit in layers to help with finding the right lift and can even install bearing spacers to provide the lift - the rubber sheeting provides more noise reduction and some level of comfort from noise you just apply that sheeting to where the upper shock mount bolts mount to frame and sandwich those rubber sheets between the frame and the spacer for each bolt - use a thread length suitable - their pitch? May vary from year to year for they "stamp" and die in a thread that can change in model years as they have made several changes to their body styles between 2011-2013/2014 years.
The Front struts also will need to be refitted and may be beyond the scope of this thread but your font stock struts can be lifted using simpler hardware - for you remove the "puck" and pound out the studs and install longer bolts and you can even make do with muffler flanges to provide the bulk spacing needed to lift the front nose. This becomes the more permanent solution and is not everyone cup of tea for raising the height of the nose can cause issues with sway bar throw and their stability End Links it uses and it's ability to rotate thru is range to provide some bounce against the sway or uneven pavement (again, a factor of living in Michigan rust belt - for pavement is possible - smooth pavement surfaces are a luxury).
That thread can be found here...
https://www.fordfiesta.org/threads/front-anti-sway-bar-end-links-choices-of-types.8797/post-23708
There are methods but there are the drawbacks to each one, for the air suspension kit is great and for most cases can be left installed as needed for camping - not sure of the long term life expectancy though. Part of this thought was to help make the Fiesta I own
- - the lifting method I used, more permanent and have encountered very few problems with this
- - for if you go too-high in the back, you're nose can scrape and you can get a nasty road rash and even suffer fog light and headlight damage from the friction and force of the bottoming of the nose due to the rise angle from the rear - Radiator and AC condensor can also get jolted from this.
- - If you go too high for lifting in the front you'll hear clanging from the sway bar and it hitting the top stop (over rotation clearance issue) and if you're not careful with height - the sway bar end links can also be damaged.
- So I chose a given lift of height, made the mods for the front struts and use the rear spring upgrade and - made the change permanent.
So; if you choose the Air kit, you just install and ride with it and adjust as needed - just not sure of how long the system will stay sealed to help you along the way as the miles add up. The concerns I had with airlifts were from the trailer and load shift on the tongue weight and how the vehicle will rock or bounce (to a point at times you may want to take Dramamine to help with motion sickness) when you travel on bumpy roads and entry into an out of parking lots - you may have sway control on the fronts, but you didn't have sway control from front to back to help reduce the bouncing and load shifting when you accelerate and brake in stop start traffic or in emergency stop conditions while trailering.