It would help the heat build up hugely if you wrapped your primary pipes, other than that, as others have said, experiment with ducting in fresh air from the front, and stick heating reflecting insulation on the engine side of the bulkheads, which is a really fiddly job, even with the engine out, or put more insulation in under all the footwell carpets, not forgetting under the bulkhead. Anything to prevent the heat soak from getting to the cabin.
