I made this custom of Rattrap amny years ago. The only reason he's making the site now is because I came across some old pictures I took of him at the time. He started out as the Walmart exclusive figure Packrat. I completely repainted him in more show accurate colors and reworked his body. I began by separating his robot body from his rat body. Next, I used 2 part epoxy and filled in his back where some of his robot parts went in his rat mode. I then took the back half of the rat and used 2 part epoxy to secure a nail in the underside. After that I drilled a hole in the back of the robot to secure his rat half when in robot mode.
The next part was tricky. I had to remove the ball joints that held his rat head on. I cut the ball joints at the ball and drilled two small holes in them and inserted pins in them so that when he's in robot mode. The rat head halves can sit on his shoulders. Keep in mind, when reading this "how-to" that I created this figure at least a decade ago and I'm trying to remember how exactly that I did it. To attach his tail on Ratmode, I drilled a hole the same size as the end of the tail so it could attach.
So here he is. I'm sorry for the crappy pics. These were shot with a 35mm camera.