Even though you're scratch building, follow the PML construction tips. You have to really rough-up the bonding areas of the QT. If the fins are G-10/fiberglass, you have to rough those up too. Epoxy doesn't soak in those materials like it does cardboard, phenolic, or wood. A good joint between the motor tube and fins is very important.
Early in my rocketry life, I built several rockets with hobby epoxy and most all are still flight-worthy and none experienced epoxy failure that I can recall. I would recommend at least 15 minute to give you plenty of working time. Once you gain experience, 5-minute will work, but you have to plan ahead and be quick. I don't use hobby epoxy any more - the small bottles aren't really cheaper than a quart or larger sized quality epoxy like Aeropoxy or West Systems. I've built several large rockets that would probably take an entire set of hobby epoxy bottles and I like working with the much less viscious epoxies like the high quality products.
As others have said, if you use hobby epoxy, don't use fillers. If using one of the high quality products, chopped fiberglass or carbon fiber will strengthen the joint. Those products with fillers are a bugger to sand, so plan ahead and shape the fillets as precisely as you can. Learn to use alcohol to smooth and remove epoxy. System 3 has a very good resource called "The Epoxy Book" that describes the usage of the various fillers. It is worth reading.