I would consider a title table for the row levels within the Crosstab Corner.
1). Unlock the report
2). Drag a Block inside the Corner
3). Drag the measure text item into the block
4). Set the block's bottom padding to 5 px (gives a nice space between this and the new titles)
5). Insert a 2 column, 1 row table beneath the Block, within the Crosstab Corner
6). Insert text items in each column per your row levels
7). Select the Outer Row edge and the left cell in the new title table.

. In Sixe and Overflow, set the width to something... I used 150 px
9). Repeat width setting for both the inner row edge and rightmost cell in title table
This should give the desired look. See attached.