Hello Dakkanauts.
I just constructed my first Onager. Beautiful model.
This is my way of making all of this kit tablewise. So without any additional waffle:
If we want to keep 3 (out of four weapons) easily exchangable, you can do this:
- glue all options you want:
Our options:
- Neutron Laser (
NL)
- Eradication Beamer (
EB)
- Icarus Array (
IA)
I've dropped on twin linked Heavy Phosphor Blaster (HPB) because of my army setup, but you can exchange it for Icarus Array. Onager set comes up with three mountings.
NL and
EB have one mounting each and
IA and HPB share one.
I will focus on
IA rocket box, because for others: you have enough skill to do so

just drill a bit (2mm wide neodymium magnets will do great) put magnets between gun and turret. Remember how to align S/N. Done.
1. Make sure you DO NOT remove inside sprue from "rocket box". It will make beautiful mounting for magnet. Later, take two sprues and glue them together (widest surface).
2. Glue the magnet (citadel thick superglue) to one side of glued sprues.
(bottom magnets are just to keep top one in place)
3. Glue that thing you have made to the bottom of the Onager "sideways turret". Magnet up.
4. Keep in mind that this turret has really nice mounting

TY
GW!
5. Glue magnets RIGHT WAY UP to the "rocket box" and "mechanicum" cover.
6. Congrats!
This is what I used and made:
1. Thick Citadel Super Glue
2. Magnetised rocket box (I love how you can make it open and close)
3. Magnetised "sideways turret". How should I call it? Is it sponson?
4. Magnetised Cover (instead of "turret box")
5. toothpick with bluetack for gluing magnets. Good thing that bluetack and toothpick wont affect magnetic field
Cheers!
/edit: pics hidden under spoiles.