There are so many ways of constructing green crab traps that it can boggle one's mind. This is a very simple one that can be made quickly and works fairly well. It is certainly not the best one I have ever used, but it is easy to put together and for the little time and effort you will spend in putting it together, it will repay for itself many times over. Nothing is critical about its size as dimensions can be changed to suit your needs. Any of the traps I have built are what I consider to be non-conforming, i.e. they will catch like crazy, but I do not have any idea if they would conform to any state law requirements or even if your state has any requirements for green crab traps.
Think of an over sized shoe box. This is the basic shape for one of the most simplest traps to build. The best material to use is vinyl coated wire mesh of marine quality. However, just about any material you have laying around will work, only the longevity of trap will be compromised. If you do not have a metal bending brake, clamping the mesh between two boards and the gentle use of a rubber mallet works good enough. While you can put the wire mesh together with bailing wire, by far the best tool for the purpose is hog ring pliers. They can be purchased for around $10 see Hog Ring Plier Straight 3703 | eBay
Use stainless hog rings of the appropriate size which should be purchased by the pound or in bulk rather than say five hog rings to the bag. Many local fishing supply stores carry them and if you end up paying more than 2 cents per ring, you are paying too much. Now for the trap itself:
This is a very simple ramp trap. It has the advantage of a wide opening which cannot be blocked by an over sized crab or a small conch as can happen with using an eel trap for catching green crabs. Several crabs can file in simultaneously. The only minor disadvantage is it has to be dropped down as shown. If dropped down on its side it will still work but you would no longer have gravity working for you. The opening between the top and bottom ramp is not critical but something around 1.5 inches more or less depending on the size of the crabs you are targeting. Usually I try to have the opening being favored nearer the top only because it can hold more crabs that way for a given size trap. If you should place this trap near an exposed shoreline, it can walk away on you with tidal or wave action. Rather than having a weigh inside of the trap I always preferred to tie a float line to a brick and then the trap to the brick. You could could add a second trap to the same brick as well. The bigger the trap, the more crabs it can hold. If you have some used zincs that you are going to toss away, drill a small hole in them and use bailing wire to attach them to your trap. This will add years to the life of your trap. I always have had the entire end of the trap open, hence the entire back of the trap becomes the door. Be sure to leave about an inch overlap for the door on three sides (not the hinge side) since this is not a precision job. If you make a very large trap then perhaps you should give some consideration to reducing the size of the door. For example, if your trap ends up being 18 inches wide by 12 inches high, and you are trying to dump your catch into a five gallon pail, well you get the picture. By making the door exit smaller, you will not have to chase spilled green crabs running all over your deck. While most use the hog rings for hinges, a better approach is to use scrap bungee cord for hinges. Simply make a single turn of bungee cord where you wish the hinge to be and then close the bungee cord with a hog ring. I do not like to use hog rings for hinges since after extended use the stainless rings will cause excessive wear on the wire mesh. When I get some time I will edit this post and add a picture of the correct way on how make a metal clip for the door's bungee cord. I have never seen this done correctly which is somewhat surprising since it is so simple to do properly.