If you want the heater in the tub and fed through the side, I see no reason to run the exhaust out the side of the tub rather than straight up. Straight up, perhaps insulated, offers the least amount of drag on airflow, and would allow for a startup bypass between top of riser and chimney. Then once the fire is going well, closing the bypass routes the flow down through whatever bell or duct you decide on.
I would locate the riser enclosure (propane tank(s)) as close to the tote wall as possible while leaving some water space for safety, so the feed tube can be as vertical as possible, and allow a bit of flat burn tunnel. I think this will help the combustion effectiveness. For that matter, you could even make a larger (say 12" w x 20" h) metal plate in the tote wall and have a proper J-tube, with all insulated vertical feed outside, burn tunnel through the flange, and riser in enclosure a couple of inches inside the tote. Dimensions like 12" feed, 18" burn tunnel, and 36" riser (measured along outer edges of flow channel) would probably work very well.
Another idea would be to mount a "bell" into the floor of the tote and put the insulated J-tube under that, so the hot core doesn't need to pass through the tote wall at all. Run the chimney from near the base of the bell, either straight up or around the floor first and then up. Elevating the tote by a foot, or digging down a foot for the core, seem like trivial tasks.