I would go with the Buhor trainer wing now that I look at it more. Finding the 2mm coro could be tough locally. I found mine at a local sign shop supplier. 4mm coro is quite easy to find though at any grocery store, sign shop, convenience store, beverage dealer, the list goes on. If you tell them you are making a plane with the stuff, most will give you the coro. You could easily make the wing at the dimensions you stated. To get a spar right at 1 3/4 inches might tough with out a saw, but the heavy yard sticks they sell at HomeDepot in the paint department and the thickness of the coroplast, you might be right there. Or close enough A little thinner spar will just make it a bit faster is all.

Plastic is a little heavier than Balsa, but it sure takes the abuse better. I've smacked my wing I don't no how many times and she's still flying.

Good Luck,