I’m not going to say you are wrong about diagonal weapon placement, because it is a popular layout for a reason, but for me personally it’s never been the best option.
Diagonal means that you can shoot sideways, forwards, and backwards (which is good), but also means that you are blocked in one diagonal direction forwards, and one diagonal backwards. I rarely find myself wanting to shoot perfectly sideways, but I am shooting diagonal almost all the time. Giving up two diagonal approaches just isn’t worth the advantages for me.
There are some other disadvantages as well, but I’m going to skip them and focus on the bigger picture for gun placement:
No matter how you place your guns, they are going to block each other in some situations. You need to figure out which situations are the most important for your build and playstyle. There is always going to be a tradeoff.
When I’m using either of the drift cabins, I find it better to use different elevations and put them in a row. One gun goes on the roof, the next on the hood, and the third one in front of the cabin on either the frame or on a buggy floor (depending on the height of the weapons).
With that setup, you are only blocked from firing backwards, and are free to drift back and forth while tracking your victim with your guns. It also means that a single cannon shot won’t hit all your guns at the same time.
In some cases, you may want to use larger wheels on the back, to allow you to put armour in front of your guns without blocking your downward angles.
There are lots of viable approaches, so the important thing is to do a lot of testing in the garage to figure out where your blind spots are, and also whether your barrels are blocked from pointing down (very annoying on hills).
If I don’t want to have a long build, sometimes I will mount two guns to the back of the roof of the cabin on the corners, and a third on the hood in front of the windshield. Or if the gun is tall enough, mount the rear ones to passthrough parts one block below the roof the cabin to protect them better (this is where tilting the build with different sized wheels is sometimes needed).
Four gun setups require a bit more inventiveness, but the principles are the same.