I recently finished insulating the interior. At first against noise and now against extrem temperatures. Important for the insulation material is the fact that is does not trap water. The only option I found – specially when you have only limited space and no straight walls to work with – is closed-cell foam. After some research I found Neoprene has one of the best R-Values in this category. Unfortunately it is not the cheapest material either.
In the end I picked a mixture of materials for different applications (windows, panels, headliner, floor). Every possible area is now covered with either reflective bubble wrap, Eco Foam or ½ inch High-Quality Neoprene which should keep the interior at a decent temperature on hot days and frosty nights.