A heat exchanger is a system that move thermal energy from one flowing substance to another flowing substance. You can find heat exchangers around your home in your steam radiator, your fridge and your hot water heater. A lot of heat exchangers use a hot liquid and cold air as the two substances, though air to air and liquid to liquid systems work also. The easiest, and most versatile, air to air system you can build yourself involves twin coils of copper tubing.
Hold the two pieces copper tubing next to each other with the ends even, against the small tubular insulation so that all of the tubes are parallel. There should be about 3 inches of tub sticking past the end of insulation.
Use duct tape the secure the copper tubes to the insulation at the end of the insulation only.
Twist the tubes slighting then bend them around the insulation. The tubes should be tight against the insulation and in constant contact.
Continue bending the tubes around the insulation in a coil until you reach the other end of the insulation then secure the end of the tubes to the insulation using more duct tape.
Wrap the large diameter insulation around the outside of the insulation-coiled tube system and secure it in place using duct tape.
This system will have limited flow capabilities. If you need additional air flow, connect multiple systems in parallel.