If you want to be super super sure fit a copper rad, loses temp a lot better than alloy.
It is quite normal for the cap to hiss slightly, after you switch off the coolant gets hotter absorbing heat that isn't being dissipated by the rad due to water pump having stopped circulating water.
Panelling in can be done with thin ply, but make sure you cut ridges in it east west wise in relation to the car, so in the even of a light bump it doesn't go through your rad also and ruin that, costs nothing but a few mins with a saw to cut some grooves, but potentially could save you a few hundred.
tabetha