More info about my design and its strength. Small bolt holes are 10,5mm and bolt holding uniball is 20, so there is very little "slop". Actually there is M20 thread on the inner part.
I did few strenght tests with solidworks simulation tool, using 6082 T6 material.
I have chosen Hirschmann M20 joint, which can stand 4500kg static load and 9000kg dynamic load.
Small bolt holes were used to "fix" the part and "force" from uniball bolt hole. Force is 2000kg per on 20mm hole, force direction to the back.
Inner part factor of safety is 3,5 and outer part factor of safety is over 5. Putting these two parts together should make a strong housing for uniball joint.