Dave before you go hacking into the hull, from experiance of a similar repair on my Stealth ( hit a submerged fishing pot line with the dagger board whilst at top speed with the spinny up, nothing is going to survive except a kick back dagger board ) its far far far easier to fit a complete dagger board case and then do the local repair at the bottom from the outside.

Get a very sharp screw driver and slowly prise the dagger board case away from the hull by chiselling off 1 or 2 layers of the trunks glass layers starting from the middle section, you will quite quickly just delaminate the damaged trunk away from the hull leaving a very neat pefectly intact top section and 3/4 of the bottom section to simply slide in the new case and epoxy in. Then slide in and epoxy in a new repair section where the damage is on the bottom, fill with foam and then lay in a suitable chamfered in top layer.

If you need further help or questions then PM me.

JP is the master in this as he has done many similar repairs and has developed a repair method which is quickest and strongest, he will give good advice.