How should gutter downspouts connect to a french drain system?

When we recently had a house built for us, the contractor directly connected the downspout into a 6" french drain system with the aluminum downspout screwed into the PVC end cap with a rectangular connector the size of the downspout. I had always worried that leaves and other debris would come thru the downspout into the French drain and clog it, which would be especially troublesome during winter. A TV show suggested that instead of the PVC endcap, the entrance to the french drain should be a catch basin with a grate and there should be an air gap between the downspout and the grate. That causes the debris to fall onto the grate while the water goes into the french drain.

Trouble is the 6" drain pipe is too close to the house foundation to put a "centered" basin and grate and I can't find one that is offset to one side. The debris, therefore, does not go into the French drain but the centered grate is not far enough under the downspout to catch all the water.

Does anyone know where I can find a catch basin and grate that is offset to one side? I have not found any. Thanks.