I have a 25 gallon fuel tank that straps into the back seat. It uses the passenger seat belt and shoulder harness to secure it in place.
An 18 inch diameter soft duffel bag usually sits on top of the tank and neatly occupies the space in the canopy.
Tool box, bags, boxes, etc all fit in front of the tank between the tank and the front seat bulkhead. I usually place the hard objects on the floor first, so they will not interfere with the speed brake operation.
I also have a Velcro strip on top of the tank that I mount my Ham Radio antennas on.
Use extreme caution with any bags, boxes, etc in the rear seat. My rear control stick is removable, but regardless, I pack the rear seat and secure everything to minimize the risk of objects shifting around and jamming the control system.
I made 2 cloth covers that cover the strake cutouts in the rear seat. These Valcro in place, and keep anything in the strakes from falling into the rear seat. Its a real bummer when you reach for your sandwiches, only to discover they are now resting on the back seat, and are out of reach, even with the "Retrieval Rod".
I then place anything I may need during the flight in those cheap plastic bags they use at almost every store in the country. I have a metal rod with a small hook on it (thick cloths hangar about 18 inches long) I use to retrieve these bags out of the strake or from the rear seat.
I keep the "Retrieval Rod" in the plane all the time. It tucks away beside the Front Seat cushion.
Don't forget to blow air through the relief tube when finished. This prevents residual fluid from freezing in the tube. I clear the tube, I simply open my outside air vent, and hold the tube up against it for 30 seconds.
Good Luck
Waiter