Don’t do it
If you take the Defender as an example the tow ball is only a couple of inches from the rear of the vehicle, this is not done to make life awkward and skin your knuckles, it is done to achieve the best ‘D’ value, in this case 17.4 kN. The further you move the tow ball backwards the more you have to reduce the ‘D’ value and in effect reducing your towing capacity.
The difference between theoretical ‘D’ value and the permissible value is set by the vehicle manufacture who takes into account things like weight transfer, rear axle load, handling and steering behaviour. 50mm rearwards movement of the tow ball will have a significant effect to the point where it is possible to make a vehicle jack-knife when conducting identical tests before and after.
The ideal set up for a tow bar is to have the ball on the rear axle which is not practical on most cars, however it is achieve in HGV’s. Make this change and have accident and you will be liable if the change is considered to have contributed.
I think you should change the draw bar on the trailer, who made your trailer? Some manufacture can supplier longer draw bars that you bolt on above the existing bar.