Here is one I made from a 45 Imperial gallon drum and assorted junk on hand.
The paddles (2) were straight and extended out about 3-4 inches. I found it worked best at about 45* so the scooped up concrete would drop straight down when the paddle reached the 12 O'clock position. Your speed looks about right to do that. I used a two stage pulley (old dryers) reduction system and it worked OK at first, then the belts began to jump off when it was called to work really hard. I ended up finding a worm reduction box to solve that problem. That is the only picture I have and I left it behind in my last move.
Yours looks OK, I like that inner bucket set up.......makes both transport and clean up easier. One change I would make.....put a soft coupling between the motor and that gear box, otherwise the bearings will go on one of them with it bolted down rigidly as you have. No need for anything too complicated, a short length of heater hose or similar will do.