PtQ's advice on making a coreless shield from polyplank foam:
sinse Saki gave you the heavy board advise, which isn't bad advise, i'll give you the flip side of the shield tech.
I use a coreless strap shield, i would make the shield out of a 2 inch 2.2 plank foam 27 inch shield blank with a 1 inch laminated sheet of 1.5 or 1.7 plank foam for the face of the shield. and a half inch of 1.5 to 1.2 foam on the edge (blue foam might work for that edging). with the edging it becomes a 28 inch shield :roll:
The straping is kind or complicated and you want to put them on there after you laminate the harder foam to the softer foam, so you can fix your straps if they get torn or damaged.
For the straps them self you want to take some strong fabric, trigger or duck cloth. take a 5 inch by however long your straping needs to be and fold it in half and sew it to itself, (with a .5 inch seam allowance) will give you a 2inch tube when you turn it rightside out. Sew one end of them so the become a loop that attaches to it self. take that loop and put a chain link connector throw that loop, * the chain connector (CC) closed, super glue it if you want.
Cut a top slit in the foam and a bottom slit in your shield foam, (where ever you want the straps to be on the shield) feed the straping throw the foam, so that the strap goes from the bottom to the face of the shield and then back to the inside of the shield, send the CC throw first mind you so in the end the CC is just peaking out of the top slot on the inside of the shield, and then the tail end of the strap is flat against the face of the shield, threads throw the bottom slot and hangs loose out of the bottom slot of the inside of the shield.
thread the tail end of the strap up and throw the CC, so if you pull on it, it will come and lay flush on the inside of the shield.
Then take 4 zip ties and punch them throw the foam two on each side of the CC so that it feeds throw the CC and the top slot, then it spans out at least 3-4 inches from the CC and comes back to itself after passing throw the shield foam, and zipp them down as tight as you can to the foam. Ideally you want the zipties to be 90 degrees from one another. so you have 4 on each CC holding it in place inside the slot.
Now sew velcro to the shield straps so that it will hold the straps tightly against your forearm and your hand. i recommend making your straps tight on your arm, because i believe the tighter the straps the better the shield is for you.
Now let me explain why we use the CC to thread throw. all the impact of shield bashing, rimming will eventually tear a coreless shield to shreds. The CCs, because they are ziptied from both sides in two different directions, are unable to move from where they are anchored. The way the strapping feeds throw the CC all that stress we just talked about goes into them and because they are anchored in place all that stress dies with them and doesn't damage the foam of the shield.
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Sources for 2.2 lb plank foam:
From Fitz: http://www.calistonarmory.com
http://www.foambymail.com/Polyethylene.html
http://www.foamforyou.com/polyethylene_foam.htm
http://www.ire-tex.com/default.cfm/iQCat=/Store.Plank.htm