The Englands in real life are quite unusually built. Much of the structure of the locomotive is the boiler itself. The frames are bolted to the front of the firebox and the drawbar to the rear meaning that the firebox is taking much of the drawbar load. The saddle tank sits onto a subframe supported on a frame, with valances to either side hung off of the subframe.
When modelling the Englands, since many of the different scale kits are without chassis to provide support during fabrication/transit, the majority of the structural integrity is taken up by these subframes with a small component being provided by the attachments between assemblies (smokebox/saddle tank/cab).
The subframes need to appear delicate from the outside as they do on the real thing, yet have strong structural integrity.
Initial efforts surrounded strengthening the valances, thickening them inwards with additional supports and cross members. However in all attempts they were simply too fragile.
A novel approach has now been taken which is to model them afresh and afix high resolution prints for the valances to them. This part of the development has been incredibly iterative through the use of 3D printing with over 20 iterations leading to the final result.