Adding detail to the weapon sponsons:

I removed the drop pod missile launchers, originally installed to provide close in light infantry defence. They do rather complicate the weapon layout.

So here's an example of how it goes. Each step is easy, it just eats up time.

First, I use a cutting board to make sure everything is rectangular. The steel edged ruler USUALLY keeps edges straight.
  

In this case I used the first set of seat I cut out as a template and just cut the same lines into a second piece of plastic.

Easy. I can use the same trick in other places. For example, the rear wall of the troop bay. Here I've used a pencil to trace out the door frame.

This ladder is made using the ruler again. Then I simply cut out each section - note this is all much easier with 0.5mm or 0.25mm plasticard, and NOT the thicker 1mm stuff.

I then use the same cutouts for the steps - recycling makes things easier!

I've been using plastic rivets I found on HLJ and ebay, but you can easily use plastic rod instead (or even plasticard cut fine if you don't mind a square cross-section). Just cut the rod finely as shown here, and glue in place.

Here is some of the original concept art from a decade ago that led to this tank. I always had plasma in mind for sponson weapons as existing Astartes vehicles don't really use it much. I sourced part from Ebay in the end as I wasn't keen to try scratch-building these (though I now think it would've been easier than I'd thought).

Not too much progress the last few days. I've clad more of the rear and added a few details. I'm getting very close to being able to paint the H hull section, if only I can work out what sort of side hatches and detail I want.

And I got bored and worked on the interior a bit. Despite promising myself I would NOT detail the interior I wound up doing precisely that, at least to the extent of starting on the troop bay.

 

 Space marine seats and lockers:

Seven to a side, so at least 14 marines I guess, not counting any left standing. There's a thought, maybe I'll add a central roof console with grab rails, like a metro train. I really like that idea now I've had it!

I considered servitors or space marines on external seats behind the plasma arrays. While this would be very interesting it'd be a bit too Mechanicus for an Astartes tank.

So I went looking for alternatives, and found this little sub-assembly:

It bolts onto the weapon mounting as follows:

I like the way all those exhaust vents match up. 

Ever watch Macross? Remember all those bits where the pod doors open to reveal ALL THE MISSILES!!!!!?

To make it all fit I had to stretch the forward arm of the weapon mount, adding another hydraulic arm. Still some detail to add here, because you will actually see some bits past all the guns.

 I've made some progress on the rest of the tank, believe it or not, with steady progress on the hull sides and rear section. A lot of plasticard and rivets. Oh, and test fitting two belt fed semi-automatic cyclone missile launchers. I'd welcome your thoughts on these?

  
So the final weapon load-out probably looks something like this: