Want to build an almost free sailing skiff

I've now built a boat every summer for the last 5 years but skipped that year. Originally we wanted to do an Opti since my oldest son just learned sailing in one but we had to go on vacation so there was no time. Maybe an Opti would been a step back from my last project anyways so I came up with a new challenge.
My last boat was a PT11 inspired 4 piece boat that fits in the trunk of my Sharan without taking out the second row. My son loves my boat and wants something faster than an Opti now, I was thinking some self bailing skiff like the O'pen Bic but a little bigger. I also found the Microskifff/Nano 2.6 online and think it is a nice little boat. Unfortunately the designer is somewhere off sailing and not to be reached - good for him.
I am limited to 115cm, maybe 120cm beam (trunk) so a boat like the Paperjet is not possible (I will do one some day...maybe in 10 years).
If I am hones performance isn't really needed either. Just a nice boat that gives me much pleasure to build in the basement over this winter.
I want to try building molds and doing it from scrap HDPE. I know the O'pen Bic is done in Twin Sheet process with 10mm wall strength (or maybe the PE sheets it is formed from are 10mm). Meaning it is completely hollow.
My plan is to build molds from hardboard (4, each 90cm long max.) and fill them with HDPE using a heat gun, then fill the parts with foam and weld on the deck. I figure that I won't need 10mm material thickness since I got bulkheads and foam to stiffen things up. I've tried forming the PE into molds already, also made a small manual injection molding machine last year (I teach arts+craft in highschool so I can play with such things the whole year round.). PE should be durable and doesn't cost me anything, only need to collect 40-50kg.
The 4 parts are going to get connected just like the Reverso, the only 4 part commercial sailing dinghy I know, 2 ratchet straps on the floor in canals. I won't use them for hiking straps like they do in the Reverso.
The problem with the Reverso is that it is too wide and stacks rather poorly.
I also got a Mirage Drive so I might do the part with the daggerboard twice to have one for the Mirage Drive.
I also like the mastpartner on the Paperjet. Maybe it could be done removable.
So any ideas on a suitable design? Is there something like an open transom sailing canoe?

 

Where are you getting this free HDPE and what form of it if you can "fill" ? with a heat gun? You're going to melt and pour it?

 

https://photos.app.goo.gl/TZ2trwkgEF6g7mLY9
Everything with the recycling sign 2 on it. All PET bottle caps, tetrapak caps, ketchup bottles, lot of detergent bottles, bigger parts in the plastics trash, sometimes you find HDPE pallets.
There's also precious plastic.
Precious Plastic https://preciousplastic.com
I've meet the guy that voluntarily runs pp here in Vienna, they have collected a lot of material too and got a shredder.
I build a mold out of hardboard and melt (not completely, cantc really explain it in English...you have to try it) the material in boards/sheets and roll it into the form with a silicone roller. In the picture above I've just fused and pressed some bigger caps together. Works quite well.
Never use a gas torch, it will burn the material. I am using a regulated heat gun @300-330°C, just so that it doesn't burn or completely melt.

 

The word in English is "sintering". Interesting. I've pondering this material/technique before too. The relative weakness of it compared to virgin plastic or GRP always scared me off on it.

 

You are very brave heating Plastic.

"When it comes to Bisphenol A exposure from polycarbonate plastic bottles, it's not whether the container is new or old but the liquid's temperature that has the most impact on how much BPA is released, according to new research. BPA is one of many man-made chemicals classified as endocrine disruptors, which alter the function of the endocrine system by mimicking the role of the body's natural hormones."

Plastic Bottles Release Potentially Harmful Chemicals (Bisphenol A) After Contact With Hot Liquids https://www.sciencedaily.com/releases/2008/01/080130092108.htm

 

The recycling code for PC is 7.
PE can contain oestrogen like substances.
Bisphenol A is in epoxy and in tin can lining.
PVC is nasty.

 

Code 7
"Potential Problems: Studies show polycarbonates can also leach the potentially harmful synthetic hormone Bisphenol A (BPA). This problem is amplified when the container is filled with hot liquids or exposed to high heat such as being left in a car."
Plastic Recycling Codes http://www.h2no.org/plastic_recycling_codes.asp

 

Yes, that's PC/Polycarbonates.

The code for HDPE is 2.
From your link:
"Potential Problems: No known problems"

I use a mask when working with epoxy, I will use a mask when working with HDPE.

 

There is another PE boat.

Fusion Sailboats :: Fusion Sailboats http://www.fusionsailboats.nl

Maybe this one is PE too.
Maverick world https://www.facebook.com/maverickworlduk/videos/2070201323004419/

 

I've found some other PE boats.
Maverick world https://www.facebook.com/maverickworlduk/videos/2070201323004419/
Seems to be made out of PE

Also this one Fusion Sailboats https://fusionsailboats.co.uk