WC&PR #15 and #17 both being 24' Saloons present similar challenges, requiring resizing and mixing / matching parts from suitable kits as well as creating the larger Saloon windows.
For the novice tackling projects of this sort there's a lot of useful information about the kits, prototypes and construction in an article by Mikkel at http://www.gwr.org.uk/proratio.html
and MikeOxon on RMWeb Backdating GWR Coaches
With a good photo of #15 to hand, suitable parts from Ratio 612 (4 Compartment Composite) and Ratio 613 (2 Compartment 3rd Guard Brake) were identified
Photo reproduced with the Permission of Weston-super-Mare Library and the WC&PR Group
The middle image is of a trial mock up from photos and bottom image shows the RHS components before final gluing; rose markers are elements from Ratio 613, green from Ratio 612. In total 7 segments make up the approx 10cm side. As can been seen in this photo even after with careful cutting, the parts from 613 tended to warp slightly even with firm clamping to a supporting back strip. This was later addressed with Milliput and sanding back.
Edited by BWsTrains