I can't take credit for this idea. Several years ago I had seen a Lego Edition Ford F-150 with blocks for valve stem caps. I decided to make my own. In this instructable I am going to make two for my motorcycle. Here it goes:
Step 1: You will need:
1) How ever many blocks and which ever color you desire but they must be 2X2.
2) Metal valve stem caps.
3) Hot glue gun.
4) Utility knife.
5) Small flat screwdriver.
Step 2: Cut out the center.
Be careful while doing this. Cut out as much of the center cylinder shape as possible. You should end up with about 1/8" left since the angle of the blade doesn't allow to remove all of it in one step.
Step 3: Finish removing the center.
Now, make the next cuts in the cylinder, straight down. The cuts should be close together. This will allow you to push the pieces back, breaking them off flush with the flat part of the block. This is where you'd use the small screwdriver.
Step 4: A nicely cleared block.
Your block should look like this. You want to clear as much of the cylinder out as possible to avoid having alot of the metal valve stem cap exposed once completed.
Step 5: Fill the block with hot glue.
Fill it roughly 3/4 full. It goes without saying that you don't want to make a mess but you want to try to have excess squeeze out once the cap is inserted.
Step 6: Insert cap.
Insert the cap all the way into the block. Use the screwdriver to center and level the cap inside the block.
Step 7: Remove excess glue.
The cooling process can be sped up by placing the block in the freezer for a few minutes. You will need to first off, BE CAREFUL and second, remove the blade from the utility knife. Once the glue has cooled, cut the excess away.
Step 8: It should look something like this.
Hopefully without any blood loss.