Need advice on connecting barrel to massheater and brick flues

Greetings out of Germany!!

I've built up my firebrick rocketstove, tested it, then insulated it, and placed the rock flue tunnels, to see how things would work. I've come to a stand still now though, due to not having any kind of clay earth to work with, so I can't cob the core of the stove yet. We live in a pretty sandy region, at 0 sea level. I've located a clay site about 60 kilometers away, where a ton of clay would cost about 20 euros, about $26, but to bring it here would cost me about 200 Euros, almost $300. My husband is disappointed, that my project is getting too costly.

The other problem I'm having, is trying to figure out what kind of a connection to make from the barrel to the mass, and also for the 3 curves I need. Each of these connections will have a clean-out hole. For the curves I was thinking of using Y-tong slabs, which are easy to cut, are heat resistant, fireproof, and fairly cheap.

Here are some fotos to show what I've completed so far. I'm an over 50 yr old female, and I'm doing it pretty much all on my own, so I'm pretty proud of myself to have come so far, and if I don't finish it, no one else here will...