Oatey Mystic Rainwater Collection System, 2 in. x 3 in., White

You should look at the below product for some inspiration if you would like to upgrade your design: https://www.amazon.com/dp/B003E1RJVU?th=1 The issue with your design is that it doesn't account for what will happen when the barrel fills up. Additionally, it can't handle debris. You'll notice that the design linked has a small channel that will only allow water through, and then when the pipe is full of water because the barrel is full, all excess will route out the normal downspout which is retained. This design also has a slip fit cover which allows you to slide off the cover and clean the insides out on a yearly basis.