This "spinner" driver from Beta Tools features a sliding T-handle that can be removed for compact storage. It also lets you turn the T-handle into an L-handle for maximum torque, then back to a T-handle for quick fastener spinning. All three ends (extended end plus both ends of the handle) have a 5/16" male hex "Allen" drive.
Handle length: 122mm (4 13/16")
Leg length (overall): 250mm (9 7/8")
This size is also available as part of the Beta Tools 951AS/S6 6-Piece Sliding T-Handle SAE Hex Key Wrench Set, Part No. BT-009510770.