Super quick post today to show how I attach the weapon options to the Contemptors. I use 5mm x 1mm rare earth magnets and create recesses in the two parts shown using a Dremel and a small 5mm diameter grinding/sanding attachment. I recommend using the lowest speed setting and boring out the space for the magnet carefully and slowly as it’s easy to over do it and bore a hole right the way through the gun mount.
That would not be good and would probably result in the red helmet of censure (insert appropriate alternative Legion specific punishment of choice). You could of course simply glue the magnets to the parts as is, but that would create a 2mm+ additional collar which is fine as long as you’re cool with that. Personally I prefer this method because you achieve the same end look as you would just fitting the arm options to the Dreadnought as normal and because magnetised things are fun!