If you don't go for the delay element with ejection charge and you have the luxury to have an R-DAS or other electronic system, you can use pyro-systems. I don't know too much about them yet, but wait till my next project! Basically it is a small explosive charge with a filament in it. When the electronics gives the signal, the filament glows and sets off the charge.

The advantage is that you control the moment of parachute deployment very exactly. With the R-DAS you can set it to use the altimeter to open the main chute at, say, 200 meters. One disadvantage is that you will have to make an extra safety feature like an safe/arm plug which stops the electronics from firing the explosive while the rocket is in your hands. And you have to be able to trust this!

When you are going for altitudes higher than 1000 meters, the NAVRO demands two-stage parachuting. This means electronic deployment usually, so you better get used to it. The first stage, in this case, is a small chute or a streamer that slows the descent down to a fast but reasonable speed, the second stage is the big chute to ensure a safe landing. Obviously, the faster the rocket goes down at first, the less it will drift and the closer to your launch site your landing site will be. At the other hand you want to protect your rocket and electronics against too big shocks.

If you use pyro systems you will have to seal off the hole after the delay charge of your motor. Then you will have to fit the pyro charges in your rocket so they will blow out your chute. Of coarse, with electronic controls there is much more possible then just a piston ejection. You can design spring loaded hatches or magnetic seals or whatever you like, as a creative rocket builder!