If the coverslip if removed, various methods are suggested to position the cells parallel or perpendicular to the bloc face. A compilation of the responses follows, MANY helpful tricks will be found on these pages. I use standard glass coverslips - round ones that fit in 24 and 12 well culture dishes. I sterilize them sitting on a clean filter paper in glass petri dishes then transfer them to culture dishes and seed with cells. We have had good results with thermonox coverslips and even better with aclar provided the cells will grow on it.
Heat can curl the aclar sometimes.
I had much better luck using filter-membrane inserts. You can even polarize the cell line if necessary by placing media with serum in the well under the insert and without serum in the filter-membrane cup. The membrane then can be fixed and processed in the holder and just before embedding, separate the filter, cut it into pieces, and embed. I have used Thermanox coverslips extensively for this purpose. They are supplied sterile, and peel off easily from a warm polymerised block, leaving the cells embedded in the resin block.