The triangle area inside of the frame is reserved for my bike lock. Nothing else will be placed there. As I said earlier I'm planning on changing it to a LifePO4 battery which will make the the unstable issues disappear making most of your response irrelevant.
There's a reason I put it there. It's the only place I can put it. I may opt for a saddle pack for the batteries if I don't end up getting the LifePO4 sometime soon. But it would complicate the install as it would end up having to be separate from the battery box I use now. The front handle bars has basket mount but won't handle the weight of the batteries very well and I plan to keep that for lugging around groceries and misc other things I may need to carry with me.
I need everything to be in one location. The stability issue is not that bad. I simply slow down when taking turns. I'm not stupid.
It was designed this way to be a quick and cheap build. I don't have thousands of dollars to just toss into a website and get a mail order ebike or build the whole thing with the expensive Lithium batteries in one go. I had to build something quick and would work while I save up for better batteries. Once I do get the LifePo4s I may redesign it a bit but likely since it's working for me well enough now I might not bother.
I also lack all the fancy hardware and know how needed to build a custom box like that beyond basic drilling holes and stuff. I don't have access to power tools so I cant just take some wood and cut some down into a box either. (that and SLA batteries would NOT fit in the triangle zone of the bike anyway....did you even bother to read on what kind of batteries I'm using?)
Sorry if my bike isn't as pretty as yours. So please don't call me Lazy.