Found your post a bit late for you may be. But I have some experience with this. If you are using plastic pvc pipes go for at least 1 inch or 1-1/4 inch pipes. Otherwise they will not will hold up. Paint the pipes to protect them against UV. Unpainted will probably still last 5 years or so. There are several websites that sell many types of connectors specifically to make garden structures. Eg:
http://www.littlegreenhouse.co... . There is also furniture grade PVC pipes available - I have not used them but they are more durable I believe. This is our PVC trellis made with 1 inch PVC pipes
If you want a more durable structure use 3/4 to 1 inch EMT conduits that they use for running electrical wires. The fittings are available for a reasonable price on the net. Look for EMT canopy fittings. I made a small 10x8 structure with 1 inch tubes and fittings some 10+ years back and it is still standing intact. Even survived a 15 ft crabapple tree that fell on it. In summer it protects my veges from critters. I use a plastic fence around it with two PVC doors. In winter, with plastic cover, protects my outdoor bonsai plants.
Any questions let me know.
