opening 202 having one example pattern of through holes in accordance with the principles of the present invention the figure shows 6 holes 302 304 402 arranged around the perimeter of the opening 202 using fewer holes or using more holes is contemplated within the scope of the present invention for example if a particular spacing e g every 12 inches between holes is desired then sufficient holes to address any sized opening can be used fig5 depicts the top view of fig4 but with a sheet of plywood or other material secured over the opening in accordance with the principles of the present invention in this example only four holes were used so four fasteners 502 4504 506 508 are used these fasteners may for example be painted bright orange or red or fluorescent etc to be visibly distinct in operation they securely hold the plywood 200 over the opening but will allow its removal relatively easily fig6 and 7 depict a side view of the two part fastener constructed in accordance with the principles of the present invention the fastener has a bent bolt 602 with a top structure that fits over the edge of the plywood 200 this bolt 602 extends through the hole in the concrete slab and is threaded to match a nut 604 as shown in fig7 the nut 604 is sized sufficiently to hold a bottom sheet of plywood 702 in place as well the bolt and nut can be painted or otherwise adorned with high visibility markings to help further reduce risks of accidents on the work site fig8 10 depict a detailed view of the fastener of fig6 and 7.