rigid hull for inflatable

Hello,

I own an old Zodiac Mark2 inflatable with an inflatable keel. I would like to build a rigid hull for it. Some kind of DIY RIB.
My father built some solid fiberglass boats and some with Balscore, so i would rely on his skills, when it comes to laminating

I thought of either a solid fiberglass hull about 10-12mm thick, ore if it is stable enough, only 4mm fiberglass on the outside, one layer Coremat and 2mm fiberglass inside.

Only thing is, i thought of using the boat itself as "plug" and build a mold. But when i do this and laminate into a negative mold, the hull will be too small on the inside. So the Tubes won't fit.

Any ideas how to approach this?

Many thanks
Martin

PS: I hope anybody can understand my poor shool english