This is the "HOW To Do What You Asked" part of my answer.
To make an injection mold ,or a simple two-part mold. There are several methods, depending on how many times you want to use the mold.
The simplest method that can be done at home would be to take the pattern knife and find a small box just about 1" larger in dimensions than the knife, and about 2" deep. Rub the knife with a light coating of Vaseline. Fill the bottom of the box with 1" of Plaster of Paris and carefully set the knife flat on the plaster so it sits like it is floating. This will make the plaster a 1/2 mold when dry. Remove the knife after the plaster sets and let dry for a day or so. Brush the mold with Vaseline and set the knife back in place.Rub Vaseline over the knife. Pour another inch of plaster and let dry. The box can then be torn away from the mold,the mold split in half, and the knife removed.
To use this mold to make a one or two use casting, carve a channel in each half from the edge to the butt of the handle cavity. Make it at least the size of the butt. This will be the funnel that the handle material is poured into. Check to see that the new blade fits the mold, and make any carved adjustments needed. Lightly grease the mold halves, but the new blade tang must be clean. It should be drilled with holes or have notches to retain the handle. Set the new blade in place, close the mold and hold tight with strong rubber bands. Pour in the handle material and let cure.
The plastidip won't work well. There are some fairly firm RTV compounds,but they don't pour well. For a firmer plastic handle you could try dyed casting resin. It should pour and cure nicely.
As to the "WILL it work" answer...it will probably make a poor handle....if it works at all.
The "HOW do I do it" answer is.
Take a block of neoprene or whatever firm rubbery/plastic material you want for the handle. If the blade is a hidden tang, mortise one of the block halves so the tang just fits.(If it is a full tang, just make as any normal set of scales, using slabs of the rubbery/plastic material.) Remove the blade, tape the handle together and shape the ricasso end of the block to a curve as you want it to look. Sand it to 400 grit. To fuse the handle halves into a solid handle can be done with a vulcanizer if the material is suitable. Other methods are to use a good grade flexible epoxy and dye it the color of the handle. There is a black rubber type epoxy made for this type of bond.
Apply resin to both sides and clamp gently with the blade installed. Do not clamp hard, or you will squeeze out all the resin. Use just enough pressure to mate the surfaces. I drill the tang like swiss cheese when doing this type of assembly. I usually install pins or Corby rivets when gluing it up,too. The neoprene handle material available from the knife suppliers in 1/2" sheet works well, and gives a dark gray handle. I have been doing this for about a year now, - hunters and fishermen seem to like it so far.
This method could be done with sheet plastic,PVC,hard rubber,or any suitable material.