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I'm thinking that size magnet is also fine. I have that size and got them from Amazon.com, 100 for $8US and they are much stronger than I expected (About a pound of force each if I remember correctly) so I'd think just to hold your army on a movement tray would be fine.

Good Luck! I smell a tutorial.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801191.page http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801191.page GMT
For the movement trays, ferrous metal would work best, so maybe get hold of some thin sheet steel cut to size. That could be expensive if you don't have the tools to cut it yourself, though.
Of course, a possible solution is oven baking trays. They're thin, light, and magnets stick to them.

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They're rather shallow, so I pad out the bottom of the base with a small metal washer. The washer goes right over the dimple on standard bases. Slotted bases are tougher, you usually have to scratch out the copyright on the base to fit it in.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801827.page http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801827.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801984.page http://www.dakkadakka.com/dakkaforum/posts/preList/245860/801984.page GMT