I made one using hot glue sticks and an old medicine bottle (The orange ones from the pharmacy).
Procedure:
1. Melt hot glue sticks in a pan you don't care about (this will forever be your hot glue pan until the moment its thrown away).
2. Cut the bottom off of the medicine bottle to create a hollow tube.
2b. discard the white top and the bottom that was cut off.
3. Roll the top of the medicine bottle (the part that the lid attaches to) in the hot glue to create a thin layer on the bottle. Make sure the layer goes below the bumps that the lid attaches to.
4. Dip the bottle in a glass of water for a couple seconds.
5. Flatten out the glue to make the shape you are wanting.
6. Dip the bottle back in the water for 30-45 seconds to harden the layer of glue.
7. Repeat steps 3-6 until you reach the desired thickness.
8. Pour 1/3 - 1/2 inch of glue in a disposable glass (a medium glass from McDonalds will do).
9. Wait 20 seconds or so for the glue to slightly thicken.
10. Push the bottom end of the bottle into the center of the glass with the glue.
11. Let all the glue harden.
12. Peal away the disposable glass.
13. lube and enjoy.
To Use:
1. Stuff the tube with a rolled up paper towel (this is to stop you from pooping before your diaper gets on).
2. Lube the "toy"
3. Insert into anus.
4. Diaper up
5. Reach into the back of the diaper and remove the paper towels.
Note that if your poop is not soft, and you don't need to go, nothing will come rushing out. And if your poop is too hard it will clog.
I have it in now, and will let you guys know how it works and holds up. It was a little hard to push in given the large top and the lack of a taper (not enough room on the medicine bottle), but does not feel too uncomfortable.
All and all it took 30 minutes, and around $1 to make.