On my 350 I just went to a glass shop and had them cut two pieces for each hole. This way I just slide one half when I need to feed or do water changes. So I got it cut about 1/4" larger all the way around to cover hole completely to prevent evaporation faster.
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I'd get two pieces of thin clear acrylic sheet and mount them to the top of the tank with hinges at the rear side, that way each side can be opened individually.
You could diy by going to a Lowes, Home Depot or some other hardware store that sells plexi and just rest them on the top of the tank.
If you have ever worked with acrylic you could buy acrylic hinges and then Weldon then to the tank and plexi.
In the past I've gone to an acrylic house (somewhere that sells acylic) and bought greenhouse panels. They are cellular panels. they have an air gap between the two sides of the panel to help insulate.