mike wrote:
Or better still use a router and those circular plastic inserts that fit
flush and take the routed out piece
I do have one of these and they are very good in some circumstances,
but a) the hole is circular and b) it's quite small. I want to cut a
hatch about 2' x 3'.

How about pinning a batten to the floor as a guide, using the router
with the thinnest straight cutter to cut an oblong hole (accurate and
easy to set depth of cut) and then adding battens to support the
cutout?

Yes. You can't get close enough to the corners with a circ saw