While Meng did a wonderful job on their Tortoise kit it is based off the sole remaining example which is also the only one not to have this feature. As scratch building goes, this is a fairly complex part to get right. Fortunately, AFV Modeler
has branched out into solving this problem for you with a 2 part resin piece.
the pistol port
To be honest, when the first forum posts started appearing about this feature on the Tortoise prototypes #2-#6 my response was that it was a bit of a shame. However, it wasn't worth the aggravation to get those curves and angles right to reproduce on my what-if model. Which is why I was so happy to see this part appear for sale; cheap, looked well done, and would make a significant visual impact with minimal effort on my part. So, credit card in hand I ordered it from the UK.
Three days later, it was in my hands. Inside the package was a thick roll of bubble wrap with the part in a small Ziplock bag. Two pieces of crisp, perfectly molded resin with very small and easily removed resin gates. One thing worth mentioning is the resin itself. It is not brittle in the least and had no air bubbles that I could see. I was able to cut the gates off with normal plastic sprue nippers with no tearing or deforming of the resin which was very pleasant.
The piece itself looks the part and has no warping or defects at all. It is literally a drop in part, with one caveat. The plug itself is accurate, which means it extends into the "hull" itself. Great for displaying it open as it will be accurate but you will need to trim some of the inner plug off to make it fit on the model when closed.
To quote from the maker "the port should be located so that the bottom of the upper hinge arm is level with the top of the stowage bin assembly, with the weld almost touching it."
AFV Modeller jpg
A little bit of superglue, epoxy, or Gator Glue and you're all set.