Broken, or new and cheap, tape measure
Rubber Band
Screwdriver, sandpaper, and pliers, though other tools may be needed depending on tape measure

Step 1: Take Apart the Tape Measure

Unscrew the tape measure, or whatever else is holding it together, and take the parts out. Make sure to watch the tape curled up inside, since it has a lot of power!
Organize the parts, so you can find them all later in the project.

Step 2: Prepare the Parts

Sand down the tape measure if it seems like part of the case with stop objects from entering.
With caution and adult supervision, cut off the final inch or so. Sand until smooth to the touch

Step 3: Attach the Rubber Band to the Tape Measure End

Tie the rubber band to the small section of tape so that the part after the knot would be inside the tape measure if the tape was put back in.

Step 4: Attach the Rubber Band to the Tape Measure

In the center of one of the sides of the tape measure, there should be a post with a slot in it. Stick the rubber band in there, about half of the way down. Wrap the rubber band around the post several times, so that the band is somewhat tight when pulled to the exit of the tape measure. Once done with that, pull the tape to the exit of the case, and lodge it under it, or hold it down with a weight. Make sure that rubber band goes out of where the tape would normally go, or the next steps won't work.

Step 5: Reassemble the Tape Measure

Taking care not to pinch the rubber band, put back together the tape measure. It should look about the same except the tape might look a bit crooked, and some of the rubber band will show.

Step 6: How Do I Store and Retrieve My Stuff?

To put your things in, first, pull the tape out, keeping it away from the entrance with your hand or a weight. Then, put what you want to store through the slot. Since the slot is fairly thin, you shouldn't have to worry about things falling out. Please note: while the picture shows the tape being pulled down, that was just so I could take a picture. You should pull it upwards.
Unfortunately, as a trade-off of the tape measure's security, the process to get everything out takes a little bit. Simply take apart the tape measure, making sure to leave the rubber band mechanism in place. Then, when you're done, put it back together

Also, if you have been concerned about the size of the slot, it's plenty large enough for a lot of things. It fits bills and all American coins but the Quarter.