While there are a lot of press designs out there, the most common are wood or metal presses that hinge or clam-shell open. Once your material is in the press with the item you’ll be molding you need a fast and preferably one handed way to apply pressure to your press. We’ve tried C-clamps and the quick adjust screw-down wood clamps, but the absolute best type of clamp we’ve found is the hand pump “quick-clamp” style. These are available at most any hardware store these days, but not all are made equal. The “Craftsman” brand seems beefy, but it wears out with heavy use. The “Rigid” brand are inexpensive for this type of clamp, but they don’t give you the clamping power you need and you end up buying several clamps to get the job done.
The absolute best brand we’ve tried are the “Irwin” brand quick clamps. You can find these online or at Lowe's. Home Depot never seems to have them in stock. Get the heavy duty that has the torx head screws holding the head together. These clamps are very powerful and give you great mechanical advantage so you can use one hand to clamp your press closed.