The title says design a RAT knife but... I feel that a dedicated survival/camp hatchet would sell like hotcakes and be an awesome combo when paired with the RC-3 or 4 in the field.
So... I present to you the "RAT-chet" or RAT "cleaver" (couldn't think of a witty RC designation...

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Handle is an RC-4/RC-5 hybrid - I wanted the size and slope/lines of the RC-5 handle (horribly presented in my drawings) but I wanted to retain the finger choil of the RC-4. I also added another choil so that one could choke up in case one wanted to use this as a skinner or hand-chopper of sorts...
There is the same type of ribbing for the thumb as in all rat knives.
Thickness is .25" as in the RC-5.
the diagonal angle by the top/rear of the head would serve as another flat surface to get a different angle for batoning.
On the length, I feel if wanted, you could easily add another inch to the body in the area between the two choils. I like the stubby cleaver look of it as is but, function over fashion
Also, the curve on the blade could be tweaked for maximum and the most productive angle for choppage - I have no idea of the suitable angle/curve.
You could skeletonize the head a bit to cut down on weight but I like that its just a slab of steel and "cleaver-ish."
This is just a quick sketch but the overall proportions and silhouette I feel are pretty much in line with what I would like to see.