Whichever tool you use, I would first drill a through-hole where the
slot will stop. I'd drill it half way from either side because it will
never emerge in the right spot otherwise (unless you have very regular
timber and a pedestal drill). Drilling the hole somewhat oversize e.g.
20mm will mean any inaccuracy in the slot meeting the hole will be much
less obvious.

I'd cut the slot as two parallel cuts up to the drilled hole. Best
possible tool is a bandsaw, a fairly hefty one. Failing that a handheld
circular saw, 2 parallel cuts on each side - then handsaw the
remainder. Remember your timber may be far from square or of constant
dimensions, so a single deep cut will produce the best result. As
others have said, for 5 posts in softwood, handcutting with a new
hardpoint ripsaw isn't that bad - even without precuts with a circular
saw.