Looking good StokerDon!!
How does the rice get into the bin, a chute from the other garage door?
I simply used 2 x 6 stretchers across the top of my bin screwed into the top plates to keep it from spreading. They give me something to stand on when needed also. Mine is 8' though, not 12' like yours, and with my sloped sides I only hold about 2 ton so your extra weight in there may need the chain set up.
This one is just a basic flat bottom. I didn't want to get to crazy with the coal bin design, if I add onto the garage, the coal bin and boiler will be moving again. So, it's just a basic box that coal gets dumped into.
To answer your question about how the coal will get into the bin...
Right through the top of that window.
My coal guy Ben should like that entry. Drive right around to the other side of the garage, put the upper section of the window down, slide the shute in and let'er rip!
I just hope this thing will hold 5 ton. Using 42 qubic feet per ton, I may need to put a few hundered pounds in the back of the truck for a while.