Seems like the peafowl breeders really prefer to incubate them on their sides. The auto turners in most incubators don't work well with larger eggs on their end, so I think if I can, I'll position any turkey eggs on their sides. I studied the (very expensive) add ons for goose and emu eggs and I think I can do at least as good, maybe better, making my own. GQF makes their upright egg holders from plastic, but the trays and large egg holders are all made from 1/4 inch hardware cloth, with plastic splines like they use on report covers to cover up the sharp edges. I can easily duplicate that, or I might use the plastic needlepoint canvas from the craft store. That would be softer and flexible, which I'm not sure is an advantage when the trays get shifted back and forth by the turner. Maybe I'll try both and see what works better.
It seems like the main advantage of incubating eggs upright is that you can pack more eggs into the tray. Shipped eggs are another reason. Turkey eggs, being extra valuable (compared to our chicken eggs anyway), will be incubated on their sides.