I know what you mean, Rob. The pics below show !ONE! of my ongoing projects that started as a machine crate and is turning into a small trailer, ramp, etc., etc.
There you go Jay, nice work!
That's pretty much exactly what I do. I use what I have and if it works the way it is, I use those dimensions. After all, it's strictly for my my use so no one will ever know the difference ... except you guys of course.
The only thing I do pay particular attention to, are the stress points.
That is why I added those thrust bearings under the front wheels. I also took particular care to have the tongue built well (most trailers are solid) because it's a wagon and not a true trailer. In this case, the tongue swivels sideways at the wheel frame to move the wheels with a tie rod. Then in front of the tongue, the handle part swivels up and down for the handle or trailer hitch.
Here are some photos that show how the tongue is fixed to the wheel frame and tie rods. I split a cross tube piece to put one part on top and one part on the bottom of the wheel frame to (sort of) balance out the pulling force on it. The second photo shows only the lower one, but the 3rd photo shows the tongue with the cross tube on top and bottom of the wheel frame like a clevis or yoke. The pin goes through both of them.
Then at the handle end of the tongue I made a pivot for the handle portion to swivel up and down.
I used 1" diameter leader pins as pivot pins for all those connections.
Each of those pins are drilled and tapped for greasable Zerk fittings too.
Other than that, it's just build it with what you got and make it work.