We have two very leaky, single pane windows that we need to insulate. Because interior shrinkwrap storms are impractical for several reasons, we're looking at exterior storms instead. Home Depot has quoted us $122-$177 per window for LARSON storms that are in the size range we need - i.e. narrow but tall.
My question is where I should mount them. Simplest would seem to be an outer-mount (AKA overlap or Eastern) fit, in which the storm is sealed to the frame around the existing window. The problem with this is that in this configuration, there is no sill for the storm to rest on. All of the documentation I have found about installing storm windows talks about resting on the sill, so I can't tell if not having one available is a problem or no.
If we do indeed need a sill to rest on, and therefore are forced to go with an inset-mount (AKA blind-stop, flush or Western) it's not clear to me how I will compensate for the fact that the inset is heavily sloped and non-level. Where would I take measurements from? Depending on where in the inset I measure from the height measurements will vary; close to the edge it will be taller, as it slopes up to the interior window the measurement would be shorter.
Given this window, then, which mounting style would you use and why? Do I absolutely need a sill to rest on for an outer-mount?