We found a camp chair works well if only two people are onboard. Driver sits on the starboard tube, and drives with the tiller. Using motorcycle type tie downs we tie the chair down in four spots, to mounts I had already put into the boat. Works great on calmer days for a passenger as the mid point is a somewhat comfortable ride. We started doing that when my partner couldn't walk and was on crutches, but we still wanted to get out and do activities. On rougher days the tubes are the best place to be.
If your Zodiac is anything like ours, it will have the place to mount the seat already glued on. You could easily make a seat out of wood, and cover it with some vinyl and padding, or mount seat backs to the wood.
Read this thread for probably your best option. zodiac modifications
I have also seen some people just use a padded cooler. Serves two purposes.