Hmmm I use SMGs - I can't aim good with rails and plasma.
So replace the rails with smg on the cruiser. It's durable enough to work out well enough.
Since I don't think you can edit a rental I'll put up some pictures later showing how I put it together.
Since you can't edit a rental unless you buy it with GC you won't be able to switch my Rail cruiser over to SMG, so here's an outline of the structure I used.
In the center here (red and black) you can see that the frame is hollow. There will be connections at a few points that join one side to the other, but generally I try to keep the high value targets away from one another. On the right you can see prisms woven together, and on the left you see a support structure added temporarily in order to build the weave.
The purpose of the weave is to redirect damage away from your gun mount. Since damage only spreads to other blocks through flat faces this type of structure tends to work the same as the skin of an onion. The enemy's shots may break through the outer layer but there are still more layers behind it. It also allows you to plan the direction the incoming damage will take instead of losing a round chunk as you would with solid blocks.
Building a weave is fairly simple. First you need to lay down a support structure because you can only build on top of flat faces. (white is the easiest to see when you're removing them) Then you make little triangles of two prisms together, keeping in mind not to line them up in rows.
As you can see in these pictures, when you lose a gun the damage will be transferred to one side of the inner frame. It will spread vertically and somewhat into the woven section, but the mount on the opposite side and your exterior armor will remain intact and undamaged.
Wheels are your lifeline. Without them you'll only live long enough to see the first bomber approach. Notice in the picture their connection point is both armored and isolated from the other wheel. If you were to draw the shortest line you could through connecting blocks from one wheel mount to the other you wouldn't want it to be any shorter than 5 on-tier blocks. (match the blocks to the level of gun you're using) You also want to keep them isolated somewhat from the main structure so when you lose a wheel you won't lose much structure.