Thanks for the replies.
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Yes, the battle at La Fiere did include an armored counterattack, but from what I've read, including the Missing Lynx piece mentioned by Sean, no tank got much closer than a couple dozen yards from the bridge. At 20" x 24" the base is already pretty ponderous, and trying to fit in just the closest knocked out tank, tempting as the thought may be, would only make it even more so. That leaves me with the Opel, which fits pretty nicely into the accounts of the battle as well as the scope of the diorama.
Sean, as for the rest of your very relevant points, I've been studying the period photos of the area and planning how much foliage I'll need, as well as the level of the river. I also have an MK concrete telegraph pole in my collection of accessories for the base, and drew much of my inspiration for making this from Bob Murphy's excellent book, which I highly recommend to anyone reading this post. I know that most accounts say no German got more than 20-25 yards from the American positions, but I was thinking of adding a couple of scared and isolated German troops huddling behind the truck just for dramatic effect. A bit of artistic license, if you will, but their inclusion isn't set in stone.
As for the scene itself, I tinkered a bit with the layout and am a little happier.
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I rotated things about 90 degrees so that the American side of the bridge would be on the wide portion of the base. This decreases the size of the open area bounded by the bridge and the path and increases the amount of space available to show the defenders.
I also assembled the basic structure of the bridge.
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I made paper templates and traced them onto thin sheets of Cellfoam, then cut out the pieces and assembled them with Gorilla Glue and finishing nails for pins. This was made slightly complicated by the fact that the Cellfoam sheets weren't quite long enough, but that was easily overcome. Also, your eyes aren't playing tricks on you, the road surface does have a downward slant, which is present on the real bridge. Next up is covering this structure with cork stonework, which will cover all those nails and smooth over the rough edges. I'll have another update as soon as it's ready.