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Very very basisicly and over simplifying.
You need a decent chine in the rear 1/2-2/3rds hull lenght...the bottom of your pontoon is your chine in a pontoon boat, it is generally a reverse chine to...

I would be seriously looking at getting a kit or at the very least plans....There is a few options in the 4.2m sort of size.
I only scratch designed and built mine because it was for a specialised...

I built a small one...build thread on here somewhere.
What type/ size of boat are you thinking of building?

https://www.nzhuntingandshooting.co.nz/f18/pontoon-dingy-build-34548/