My random thoughts...
The 1/4" elastic seems thin to me. The one rocket kit that I have of comparable weight and which came outfitted with an elastic shock cord used 9' of 1/2" elastic. The ability of the elastic to absorb the shock of ejection is based both on length and width. On pyro rockets you need it long because it is common on shock cords for the ejection to stretch the cord to it's limit and then have the nose section spring back and impact the body. In the days of balsa cones this would result in a smile shaped indentation, aka the Estes smile. Now, it results in a bent body tube. The common wisdom is to make the cord twice the length of the rocket. Many will say longer than that.
Be that as it may, I doubt if you need it that long. I have no real basis for this, but I think I'd go with a shorter piece of thicker elastic. I am wondering how you will attach the elastic to the body without a 'hard point' and am worried it will rip free. Maybe thicker cord will help, maybe not.
You could always test it with a 2 lb weight. Attach it to the cord and chunk it off a deck (or whatever). That would at least verify it won't snap. I don't know know to test the attachment point.
Hope this muddies the water
