half of a tenth of millimeter brass foil that I bought in an artist shop, but they are sold also in modeler's shops or, and this is the cheapest solution, in shops where big machinery pieces are sold, they have rolls of brass foil and they call them, IIRC, shims... they use these foils to 'shim' under the feets of electric motors or pumps to center and align them..
I imagine that also a 1/10 of millimeter would do, but I haven't tried..
I cut it in the shape of the panel (just slightly bigger, simply for better handling), draw with a soft pencil or thin felt tip the louvre cuts, and then, on a hard rubber surface (so it gives way just a little), I cut the louvre's cuts with a brand new razor blade.. the 0,05 brass isn't harder than plasticard to cut...
Then I push along the cut on the side to be opened with any kind of tool, a strong toothpick or similar..
After that I trim the piece to size with sharp scissors...
Sadly ATM I have no model to show for this, but, believe me, it's easier done than said.. and looks great!