axle location
Consider make the axle on a separate (subframe) assy that can be moved back and forth and bolted in place after axle placement is determined. Plan on making a couple test tows with you normally loaded trailer, try different axle placements, fab up fenders when location is fixed.
I've seen this on boat trailers, it allows the them to make a generic trailer. Depending on the boat's weight distribution, the axle can be moved to the ideal location.
Remember a trailer with the wheels far back will be the easiest to back up, and will tow the best at highway speeds. Put the axle too far back and you'll have too much tongue weight.