I've recently purchased a laser cutter and found that a lot of sheets I was trying to cut were either warped before I started cutting or warped during cutting. Googling, I couldn't find a solution that did what I wanted, to clamp the sheets flat to the cutter bed. Using these clamps have solved the problem for me. I hope it helps you as well.

Supplies

You will need:

Pliers
Wire snips
A small, flat head, screw driver
A ruler
A small spring like you find in popup pens or in an old plug (in the UK)
About 8 cm of wire. Thicker than a paper clip but thinner than coat hanger wire. I used some copper from a high amp cable
A screw connector block. (Just one connector cut from a block of many)
Laser cut clamps from the template files included (either clip.png or clip.svg). I used 3 mm ply, but any material that is safe to cut and about the same thickness should do.
You are probably going to need to make three or four of these to clamp down your sheet.

Step 1: Cut the Wire

Cut a length of wire about 8 cm long. This was the length required for my laser cutter honeycomb bed. Look through this Instructable to judge if it will be the correct length for yours.

Step 2: Extract the Connector

Remove the metal block and screws from the connector block. The plastic part is going to be thrown away, so you can (safely) cut it up to remove the parts.

Step 3: Add a Bend

Bend the wire at about 5 mm from the end to an angle slightly more than 90 degrees to make a slight hook.

Step 4: Put It All Together

Thread one of the laser cut clamps onto the wire through the small hole in the clamp. The smaller clamps are useful where space is limited or you just want to clamp down a corner.
Thread the spring onto the wire
Screw the connection block onto the wire about 2.5 cm from the clamp
Bend the wire above the connection block to 90 degrees in the opposite direction to the hook
Your done !

Step 5: Finished

Your completed clamp should look like this.

Step 6: Use

Push the wire though the laser cutter honeycomb bed.
Pull back the wire at the top to clip the hook under the bed.
Push the wire at the top down and forward to release the hook.

The reason we bent the wire at the top the opposite way to the hook is so we know where the hook is clipped under the bed to release it and it is out of the way of the cutting head as it moves around, especially at the top of the bed where there is not much space.