That's all I do...
I cut the back piece first, adding 1" to accommodate 1/2" for each side piece.
Then when I cut the side pieces the back will fit snug and the corners will be flush.
I then attach the side pieces and tape them (OUTSIDE, NO TAPE EVER INSIDE) to the frame of the tank, top and bottom, front and back.
Then tape the back to the side pieces.
For the top, I cut it to fit and then trace my heat lamp in position where I want it to lie. I cut that hole out plus about an inch all the way around for ventilation and so the lamp isn't in direct contact with the foam board.
Voila, done. You can always paint it or whatever if you want to pretty it up. The cleaner the cuts the better it will look too.
Just be sure to let it air out for at leaat 48 hours if you paint it though.
OP, Let me know if you have amy questions.