Electrical – Are there octagonal box extenders that are slimmer than 1 1/2″ deep

I agree with answer #1 with the following exception: Do not use longer screws to attach the fixture. Find a suitable box with extensions as needed to make the box opening flush with the ceiling surface. You do not want a gap between the box and fixture the could allow heat and spark from a fault in the box to escape. Here are products that I would use in your situation. These may not be available at home improvement store but are available at electrical suppliers who service contractors.

• For support between studs -- Erico/Caddy TSGB24 or simply use a 2x4

• Box attached to support --4" octogon box

• Two extension boxes as need to bring opening flush -- 4" octogon extension box 2 1/8" deep

This combination gives you a box 5 3/4" deep. Adjust the support on the attic studs to make this flush with ceiling.

Remember that it is important to not leave a gap that could cause a smoldering fire in the voids of the ceiling structure. This is the reason that most electrical codes require that box faces be flush with the finished surface.

I suggest doing exactly what TechShop does. They hang conduit in the ceiling, and above each expected workspace area is a 120/240V twist-lock power connector. From that, a cord drops down supplying power to the workbench. (In their case a 4-outlet box dangle on strain reliefs giving two 120V circuits in an MWBC configuration.) I have to believe that arrangement is code legal, since the facility is tip-top and generally a class act, and it leases space from Ford.

You say "warehouse type space" which means commercial rules may apply, mandating conduit not Romex. Be careful here, conduits can only hold 9 current-carrying conductors (hots or neutrals, except neutrals don't count in MWBCs) before you must derate amp capacity or use larger wire. That's because a stack of tightly packed wires at rating make a fair bit of heat. If I expect to be near limit, I'll hang 2 conduits instead of 1.