I have used an expansion bit (The kind used with a "brace", as in "brace and bit", in the days when such things were done by hand).
A hole saw will also work, or a spade bit.
You can enlarge a hole by using a torch to heat up the edge and carving with a knife.
You can make an effective and inexpensive bulkhead fitting from a PVC male adapter and a female adapter with a gasket cut out of a convenient piece of rubber. You can also seal with silicone. I have screwed the male part of the adapter into a softened area of a polethylene tank and used the female adapter as a nut to seal it.
Most of the tanks that I have used have a vent in the cover. It is almost impossible to seal such a tank. Do you need a vent or an overflow?
Keeping bugs out is difficult. I have put a piece of fine screen on the vent.
You need to be sure that your attic is strong enough to support a 500 liter tank full of water. Most attics are not structurally designed to support such a large load and you could easily damage the ceiling (best case) or break the joists and have it collapse into the house (worse) or fall through and kill someone (worst case).