hotator - Thanks for the explaination. Does make alot of sense. Variation of idea may work well in my case. I found some scrap c-channel today for mounting rails. About 3" W x 9 1/2" deep, webs are around 1/4" thk. I think its heavy enough. I plan to weld stringers to set orientation with one another, then suspend/support pair above/ in hole opening. I'm not exactly sure how I'm going to do this yet, to adjust levelness and fix position. When finished with first pour, most of c-channel will be embedded in block, futher anchored in with thread rods, rebar, and scrap metal. I should be "pouring" my block and slab later this week.
Everyone - Thanks for your inputs.