As the earlier lads have intimated, you are best to fix the keel to the hull with some form of supports as a keel will probably be knocked in it's lifetime. Yes, it involves pierceing the hull but the modern glues/fillers sort this out.
And keels should be straight and run parallel to the fore & aft line.
If I remember rightly, I have about 4 or 5 'tabs' [supports] on my keels, they're about 2ft in length. I made them of 2mm ply and had resin cover them and form a fillet along the joint (both sides) and then sanded it smooth.
To form the actual keel, if you can't go off the plan, mark where the keel goes, then cut some wood/plastic to a rough shape of your keel but leave enough for your tabs on the 'inside' (towards the hull) Then it's a matter of preference if you cut the keel tabs to suit the slots or vice versa. But it's trial and error. I, myself, would cut the inner part (tabs and inside shape) first as you can then fix it to the hull. I'd then do the external edge to finish it.
But take your time and do it bit by bit, keep offering the pice up and trim bits off as needed, offer it up and trim and do it like that... and then finish with sandpaper.
Then once you have formed the inner edge, and tabs, and fixed it, you can then form the shape of the actual keel.. the 'outside' edge.
Good luck, hope it helps. Remember, it's not rocket science and you can finish it any way you want (as long as it's straight.