I am building our new personal house right now and have installed a mechanics pit in the RV garage. Mine is 3'wide, 22' long, 5' deep with eight formed concrete steps with 12" tread and 8" risers. I built the structure in the concrete flatwork stage, first pouring a large footing and then laying up block. Grouted the block solid, placed forms around the top with a 12" concrete turndown, and prepared steel for the slab pour. The verticles from the block were bent over and tied in with horizontal steel in the slab. The last step: I took 1/4" 1. 5" angle iron, mitered the corners, welded some #4 rebar 12" long, and placed them on the outside of the forms. I poured my floor, and the pit is done.
I have sourced a typical steel grate (akin to one you would find on a second story catwalk) that will be cut into segments and placed over the pit, resting on the angle iron. This way, you could drive over the pit if you needed too. Plus, each segment is only about 60 lbs, so it isn't too hard to handle.
I made sure to waterproof the block before backfill and of course run conduit for both electric and compressed air. Whole thing probably cost me $2,000, and that was with inherent builder discounts involved.
Well worth it!
My only regret: I should have built a cavity in the floor to house a sump pump.
~Chris