Thickness of acrylic

I am thinking of making a lid for my Aquaone tank. The original is similar to the rio / lido lids where the flaps connect directly to the light unit like this

I replaced the original light with a Fluval Aquasky and the flaps have nothing to clip in to, plus the new light is slimmer.
I could use a single sheet of acrylic 1000x400 mm and place the light over the top, in this case the lid would only be supported on the edges, although I would probably add a couple of dowel rods in the centre on the tank brace.
My preferred option would be to use 2 sheets at half width (less a bit for the light) so that I could make it black, and maintenance would be easier. In this case the sheets would only be supported on 3 sides with an additional supporting rod in the centre.

How thick would I need the acrylic to be to avoid bending under its own weight?

That would be a good question to put to 1st year engineering students, but they'd need you to provide the density and what is known as Modulus of Elasticity of your acrylic. But once you know what they are, you could google "deflection formula" and do the exercise yourself .

Easier option - I found this https://plasticsheetsshop.co.uk/thickness-calculator/ in case anyone needs to do similar

Hint: For horizontal placement supported on 3 sides it assumes you are supporting the long sides. So although my sheets will be 100 x 17.5 cm I had to enter 100 x 100 cm. Then I recalculated using a centre brace (therefore 50 x 50 cm) and the answer was pretty much what i had guessed - i.e. 4mm

Edit: Oh and if you are really lazy (like me) get them to put in a 6cm dia circular cutout and use a desk cable grommet as the feeding hole . Not ordered yet as I can only do this in Feb - so leaving it here as a reminder to self.