there are many good one's out there. I use the ez pole perch--very quick service and very good on the price. I think mine is 36" wide--Not sure if it will work on a jon boat but---
These hold some heavy weight--not me but the people who fish with me are some pretty big boys--for me the ez pole perch is the way to go. The only con on this type is the post's attaching the seats--if you get up you have to be careful not to trip over it going to the back of the boat but really in a jon boat you probably won't be moving around too fast anyway gl on your search.