Here you go mate, the measurments are
15cm across the top,
just over 14.5cm across the top of the gauge pods
and 5.5 cm from the top, down to the top of the gauge pod,
So should be big enough for a ACT gauge i'd think!
And there are some structure struts on the back ubt i'd think you could slide it in there!
Here's the pics next to a d500.
Front:
Back: