The Zacuto Cold Shoe Handle: Top Handle for Lightweight Cameras / Rigs provides among the simplest ways of outfitting a DSLR with a top handle for convenient carrying and shooting from low angles. Made up of a few connecting components, it mounts right into the hot-shoe of your DSLR, and once mounted, it will rotate up to 90° and will slide from side to side along 3 inches until you tighten it in the position where you find balance to be best.
The components in addition to the actual Handle are a 3 inch Z-Rail and a 90°-rotating Zotshoe. The Handle attaches to (and removes from) the Z-Rail by quick-release lever, the Z-Rail attaches to the Zotshoe by an included screw, and the Zotshoe is what slides into your camera.
Please note that although this item is an ideal solution for a DSLR since many larger-sized cameras have spaces other than hot shoes on them for attaching top handles, your camera nevertheless doesn't necessarily need to be a DSLR only. Moreover, you can attach it to more than just a camera -- you can attach it to an entire rig that your camera may be a part of, but because the attachment into the camera is into the cameras hot shoe only, any camera or rig over 10 lbs is recommended against. So, as for rigs, some suggested examples are Zacuto's Target Shooter and Striker.