My suggestions for temporary closure of the bulkhead is as follows:
1. Find in the basement two or more screweyes (see attachment) large enough
to accomodate available rope.
2. From inside the basement bulkhead screw 2 screweyes 
separated vertically by about 1 foot through the existing tarpaulin into the sheet of plywood now covering the bulkhead
3. Thread pieces of rope through the screweyes and knot the ends of the rope around one of the existing cellar stair treads
4. If necessary, tighten the knotted loop by sliding and twisting a stick of wood between the loop and the step


5. If you can't find screweyes, use a 1x2x12" piece of wood screwed under the plywood to clamp the pieces of rope.

This arrangement will keep the plywood from blowing away and block entrance
to the basement from the outside. Just to be safer, I would weight the plywood with one or more cinderblocks.

I hope that you and Laura are comfortable and well.