I haven't tried this, but if you'd like to start smaller, perhaps a DB4e could be tried, and a second added if more signal strength and/or more directionality is needed. The DB8e, from all appearances, is a pair of DB4e's with a combiner, and the gain is around 3dB higher across the spectrum over the DB4e, as would be expected with twice the antenna.
What I don't know is how practical it would be to do. It's not hard to make two short cables the same length, nor to add a splitter to combine them, but what I don't know is how effective it would be. If this is a foolish idea, I hope ADTech pops in and says as much.
Alternatively, you could buy the DB4e locally at a store that will allow you to return it for the DB8e if you don't get satisfactory results. But the experimenter in me would go for the factory antenna and DIY combining, just because.
Oh, and since Radio Shack sells (and owns) Antennacraft, perhaps try the Y5713 since it's smaller than the Y10713, as long as you can return it for the larger one at your local store (the elements are easy to fold back up, as long as you're careful to not break the locking mechanism, which you can do if you flex them too hard).
Mount them on wooden dowels hanging from rafters, clamped temporarily until the ideal location is found, then just run a couple of wood screws through them. No wind to deal with up there, so you don't need the robust installation of a roof or chimney mount.