For Clamping I bought a pile of "straight edge" normally used with plastering walls.
They have flat surface and handle to make them strong enough for clamping.
Price was with 4 dollar per meter OK.
That looks perfect
It can be any profile you want as long as it's rigid and you can easily drill it for the threaded rods to go throught, it doesn't need to be U channel or square tube specifically.
I guess it's up to each one to figure out how much exactly "hand tight" is.
You really need a torque wrench if you want to torque something. Even someone really experienced with torquing bolts with a torque wrench can achieve something like a +/- 50 % accuracy by hand only, so not really precise... ^^
Probably have one set of the 280's total 560Ah
One set of 2* 152's, total 304Ah and
One set of the bad ones.
That would be acceptable too