I just posted 10 finished I did.
https://www.dakkadakka.com/dakkaforum/posts/list/746215.page
Here is how, and it sounds hard, it was super easy.
1) 10 Cataphracti Combi Bolters.
2) Take the half of the combi bolter that does not have the fist and cut off the slide/barrell that is in front of that sides magazine
3) Take a plasma pistol and cut off the but of the gun and the handle/trigger. Basicaly right were the plasma vent ends.
4) File a little bit of the plasma pistols inside.
5) Glue the Combi bolter full half and the combi bolter cut piece together
6) Glue in the Plasma pistol piece.
I am not very experienced with converting. But I used a modeling saw to do my cutting, then fine files to get in all perfect.
Good Luck.