You could use thin rope or even string. Just use some small pieces of ply or thick card under the rope to protect the gel coat on the hull. It should not need a great deal of force as you will be putting it in the same shape as it was in the mould. I should mention that at the bow and stern the curvature will require the stringer to bend. If you cut 1/2 way thro the stringer at say every 1/4" it will then be easier to bend to shape. Just soak the cuts and stringer in glue and clamp to the hull and adjust the inside spacers to give the correct shape. Do look down the length of the hull on each side and make sure there are no obvious dips or bulges. A one meter rule is useful but there may be a flare on the plan so make sure it looks right as any mistakes will need repairing or you will have to look at them forever☹️.
You may need to raise the height of the prop shaft so you have room to fit a lower mount for the rudder. You can repair any holes with P34 or similar.
I have just released the keels from the mould so if the weather is not too cold I will join the two together and when hardened I can smooth off and they will be ready to send, hopfully next week. PM me your name and address please