After weeks of experimentation I managed some grafting on opuntia - ficus indiga rootstock. It seems that the trick is to put a lot of weight. A solution of 500 mg/lt ( 500 ppm ) seems to speed up  unification proccess a lot.
This is a very hard grafto of a trichocereus piece on opuntia stock I used petroleum gelly on the edges to help the graft maintain humidity during the healing proccess :

Here is another tricky graft . A Selenicereus grandiflorus slice on opuntia :

I have applied a mixture of lanolin paste and Forchlorfenuron about 3% to force the budding on the scion