In addition to the wife on board (directly above the offending tire) you can fill the Sunfish tub with water. 
there's still some hope—but you'll need to thread chain through the metal rim and bolt it to the trailer frame. Or use a length of fire hose and a Vise-Grip.
but you can still tow with three lugs holding the tire on.
It was not for want of leverage. The tire kept spining. I did get my wife to set on the Fish just above the axel that put enough weight on the tire, so it would not spin. But what does a feller do when he is alone?
Yes I replaced tired tire. LOLIt's spinning in place?
Best, is to spray, tap, then rest a hammer between the rubber and asphalt. (Not grass). Tow the offending lug to the 3 o'clock position.
One pipe could work, if you press down on the end of the wrench nearest the driveway [9 o'clock, opposite], so you're adding your weight to the effort.
If unsuccessful, use two pieces of pipe, and tense the muscles between the shoulders to turn it. If the metal rim spins within the rubber tire
I've broken two of my own 4-way wrenches using one pipe—at that point, you're really stuck!
The lug itself could break off,
'Course, the rim and tire can still be removed as though to re-lube the bearings.
If you can still add air to the tire, tow the whole works to the nearest tire shop. If it's an 8-inch tire that's gone flat, you're going to want to replace it anyway. While a nail is a possibility, look for small cracks within the rubber around the flattened area. (Cracks mean it's a tired tire).
Awesome Idea.
I did not try rubber chocks, but did put triangular wood chocks. Rubber more friction, yes!
You are encouraging and great advise!
It took me three times reading this, but I finally SAW your point in my mind. Thanks.
LOL LOL....I thought of that but this fat Old Man abandoned that idea.You'll have to sit on the axle by yourself.
No spokes.
I tried chocks. But like spare tire Idea...and why did I not think of that? I know I opened myself up to jokes with that comment.
My wife setting on My sunfish above the axel. Lugs came off easily.
I like the spar tire idea best. I would have had to use my vans spar and I hate getting that out. But will carry a spar for trailer from now on.
"Professional" tire shops rely on air-powered wrenches to install tires. Few operators then go back to check the proper tension on the lugs.
The entire axle was hanging in the air, as the rearmost frame had bottomed on the asphalt! Hopeless! After several failed attempts with boulders, I finally thought that since my truck didn't have a limited-slip differential, I'll put an old polypropylene rope through the spokes, and tie it to the frame. I put the truck in gear, and that worked in seconds! 
Great advise!
