what if you surround the inside of the computer with emergency blanket? wouldnt that be reflective enough and decrease the risk of fire?
I don't think it would reduce the risk of fire. Using a thermal activated off switch (a 105 F shut-off is available from SunlightSheds) would be the most effective way to avoid fire (other than configuring your system correctly). You plug the lights into the thermal shut-off and the shut-off into the outlet. Keep the thermal-switch sensor in your grow area and if the temperature reaches 105 F, the lights shut-off.
For covering/insulation, you could try using "Astro-foil" - I think that is what SunlightSheds is using in their PC Planter anyway. Nearly as reflective as mylar but easier to work with (more forgiving with wrinkles and creases). It is also heat resistant.
Something to consider anyway...