I bet if you replaced the 1x2 with 1x4 and cut it to a parabolic shape (leaving 2" of thickness at the center) like Matt's photo it would work fine (no compression, but the added thickness would most likely do what you want), just make sure you buy straight lumber then glue & screw together.
An 8' bridge will have some weight to it, but you could make it a lift out bridge if wanted.
I like the idea of making it out of aluminum, if you had a place to buy it. Maybe some 3/8" diameter rods or tubing curved into a parabolic shape screwed to some 1" angles, of course you'd have to get the bend done unless you had a way of doing it yourself. I only have about a 4 foot gap I need to make a bridge for, this may be what I use.
