I've seen a few -8's that used a computer fan mounted in the glareshield for a defroster. The only time I needed one was last summer when I departed OSH in a drizzly rain. The window fogged up while taxiing, and I used a paper towel to keep it clear. As a bonus I wound up with a really clean windscreen Once underway there was no need to keep wiping as cabin airflow kept it clear. YMMV
I recently flew on a very cool, humid morning, and with managed to fog up the windscreen and canopy pretty good while fueling, preflighting, getting in the airplane etc... As soon as the engine fired up (canopy half open) it all cleared, in seconds.
You've got lots of airflow in that situation, and in flight there seems to be plenty of ventilation.
I installed a defrost on my -6 but did not on the rest of my builds. Added complexity for just a few instances when it was needed. As others have said, it clears quickly once you start moving quickly.
Other side of the coin, I installed two computer fans that just pulled warm air from the avionics behind the panel and it worked well when I used it.
Last year's Bronze Lindy winners (partners in N53LW) added a defroster vent to their 8 using one of the three air tubes from the avionics cooling fan.
They used two tubes for avionics cooling and ran the third tube up to an oval-shaped hole formed by....drum roll please...an old aluminum Popsicle mold. It kicks out a lot of air and works fine.
If anyone has any aluminum Popsicle molds out there I'm in the market...otherwise I'll fabricate something similar.
Once underway there was no need to keep wiping as cabin airflow kept it clear. YMMV
