insulate hood panels

[the loach]

I'm going to replace the glass hood panels (20x20) on a tank, they will be acrylic with 1/8 plywood on top.
Since it is a heated tank and most of the heat dissipates through the top, looking for recommendations for insulation between the acrylic and the plywood... that could just be glued between them. Max thickness ±4mm.

 

[Cria]

pretty tough to find anything THAT thin. I'd suggest extruded polystyrene. You can buy sheets of it at major DIY stores and it comes in 1/4 inch thickness (which is 6.35 mm).
Good luck!