For simple upgrades, cleaning tasks or sighting in optics, having a way to to solidly keep your AR-platform rifle stable is key. If you're cleaning fouling out of the barrel before a match, or boresighting your red dot, being able to keep the rifle immobile to accomplish your task is like having a second set of hands.
The Ergo Grips Spikes Block for the AR-15 (a model for the AR-10 is also available) is an inexpensive option for securely attaching your AR to a workbench. Measuring 6 7/8 inches tall and 4 5/16 inches wide at the base, it can either be bolted or clamped to a workbench or other suitable surface. The magazine well of your AR-15-style rifle fits over the block, which uses the magazine catch to hold the rifle tight. Theoretically, you could bolt the block to a wall and store your AR that way if you chose.
Having a stable rest for your rifle when you need to perform maintenance or add an optic is immeasurably useful. Other models that clamp into a vise are extremely good to have, provided you have a vice. With the Spikes Block, all that's needed is a stable, flat surface and a clamp, and you've got that second set of hands you need. MSRP: $24.95