If you have a tandem axle trailer, wood screw some 2x4's togther in a pyramid with a flat top and stow it in your boat or tow vehicle. A stack of 3 on their sides will be enough lift to get the flat off the ground and the spare on if you pull the good tire on the same side up on the stack. It works great and a whole lot easier/faster than messing with a jack. Total cost if you have to buy the materials, about $5. Aggravation saved when you have to change one... Priceless!!!
I'll post a picture next weekend if not sooner