You need to appreciate the heat transfer dynamics within the engine in order to make an effective heat shield. If you remember, heat transfer occurs through 3 way - conduction, convection, radiation. Conduction which is an heated part of the engine bay or hot air touching a cold part of the engine bay or colder air. Convection which is the way gases within the engine bay and/or from the outside carrying/removing heat from one part of the engine bay to another. And radiation which is the transfer of heat via EM waves.
To your choice of aluminium sheet for the barrier, assuming the conduction and convection variables are constant, what affects the heat transfer coefficient of EM waves passing through any media is in large part the (i) colour and (ii) smoothness of the surface. As there is no light within the visible spectrum inside a closed engine bay to be "reflected" (eg. the excitation of incoming electrons to a higher energy level and being emitted as light when these electrons drop back down to a lower energy level), radiation is in this case is mainly IR. For IR, it is true that your aluminium plate on its own basis represents a shiny or smooth surface (which has no little surface cavities to trap extra EM waves) to easily "reflect" IR compared to a dull and rough surface. However, you need to consider two further points - the smooth surface of the aluminium plate can easily be replicated by a lot of other heat-stable materials, with much lower conductivity than Aluminium; and the majority of heat being transferred within the hot engine bay is mainly through conduction and convection.
While the idea of creating an air pocket is good, bear in mind that your proposed 0.5mm gap of air will easily be heated up by the hot aluminium plate because of its high thermal conductivity, thus losing all the benefits of the air insulation. Moreover, with the differential pressures and vibrations within the engine bay, it would be a challenge for thin aluminium plate with its low structural rigidity to maintain its shape and hence the 0.5mm gap (not that 0.5mm is a huge enough gap to slow down any heat transfer in the first place).
Much R&D efforts have been put into thermal wraps and heat shields in the racing industry. In your case, just take a trip down to places like Garage R or even online. You will find that most good heat shielding products are made of materials with low heat-conductivity HR foam, with a thin layer of highly reflective coating on top just to eliminate the last bits of IR. Better designs actually make use of convection, rather than a stagnant trapped layer of air, to produce an "air-cooled" box that really creates a cool reservoir of air around the intake.
Good DIY effort. Cheers.
