Based on the exploded picture on Madcow's site, this looked to me exactly like the Double Shot, the ISC is the last thing I have to work on to finish mine, I'm across the country from it right now or I'd post a picture or two. But to try to describe that one, there's a fairly long coupler that extends into the bottom of the sustainer and is long enough to extend through the entire "transition" part of the transition (i.e. down past the part where it's a steady 4"). The transition openings are sized for this coupler OD at the top, and for the 4" coupler OD at the bottom. Then there's the 4" coupler, a centering ring with a center hole for the sustainer coupler and OD'ed the same as the 4" coupler OD (so it sits on top of the 4" coupler inside the transition, and keeps the alignment on the sustainer coupler), a solid bulkplate that was sized for the ID of the 4" coupler, as well as a bulkhead that goes inside the sustainer coupler for the upper seal on the ISC.
To help with the available area for electronics, I epoxied the bulkplate to the bottom of the 4" coupler (inset just enough for a good fillet on both sides), cut the 54mm coupler (DS is 4"-to-54mm instead of 4"-to-38mm in the Nike-Apache kit) just short enough to reach past the centering ring, and epoxied the 54mm coupler to both the top of the transition and the centering ring, which is also epoxied inside the transition. So for me the separable part is where the 4" coupler goes into the transition, I figure I'll use rivets there. So I'll have access to most of the ~4x6(?)" for the electronics, though my GPS antenna will likely stick up into the 54mm coupler for some extra length. My thought was to secure the electronics to the lower bulkplate, but there will be a threaded rod that runs all the way up to (and capturing) the upper bulkplate, so that I can load the separation charge and then insert the works into the transition and rivet the pieces together for flight.