Clamp the guard securely onto the chassis rail in roughly the correct position.
Position your tyre and rim inside the guard and line up the outside of the tyre flush with the rim of the guard, both vertically and horizontally.
Holding the wheel in position (get a mate to help if necessary), measure between the hub mounting surface of the rim and the outside face of the spring - this is measurement "A"
Measure the width of the trailer for measurement "B" and measure the centre to centre of the spring bolts.
A simple way of measuring this is to measure from the outside face of one of the springs to the inside face of the opposite spring.
Multiply measurement "A" x 2 and add to "B" for the hub face to hub face measurement.
Using 50 x 50 x 4.0mm SHS, cut axle tube to length and drill the spring centre holes making sure the holes are big enough for the spring bolts.
Weld up the axle as per the instructions on "Building a Trailer Axle" page.