I love to rock climb and recognized how big a role forearm and grip strength come into play. I use a typical grip trainer but thought I could build a forearm trainer on my own for very cheap. Now the simplest way to do this would be to find a piece of wood, stick, dowel, timber etc. And drill a hole into it through the stick in the middle, tie a rope to it and then to a rock. But I wanted something strong and light, and that looked decent.
Now I decided to do this after-the-fact I made this trainer. So the pics aren't that detailed and step-by-step, but it's really simple and I trust you can do this rather easily. So, now...what you'll need:

1- 12" piece of PVC 2" in diameter
1- worm gear stainless steel pipe clamp to fit around PVC
1- length of rope about 4'
1- swivel clip

Step 1: Connecting Rope to PVC

Pretty simple, just slide pipe clamp onto PVC near as center as possible. Then feed one end of the rope between PVC and clamp. And tighten clamp with a screwdriver or hex key. Don't go crazy so you don't cut into the rope and damage it. Now I like to burn the ends of synthetic rope to prevent fraying and also it makes the end a little bigger so it can't slip knots easily.

Step 2: Attaching Swivel Clip

At the free end of the rope tie a bowline knot through the closed end of the swivel clip. And clip the swivel clip onto the rope a few inches from the knot.

Step 3: Done! Attach Any Weight You'd Like!

Now you can attach any weight you'd like to begin training. Milk jug, cinder block backpack, etc. You can clip the weight directly to the swivel clip or use like I have to create a slip know effect. I made a little film of how you use it. Thanks! And may your forearms grow crush boulders!

http://www.youtube.com/watch?v=NQfFO9os33M