Do do it yourself narrow flexible grinding block.
The base is made similar to the handle of a knife, to whom it is convenient, we customize for ourselves.
We buy or make ourselves a loop of sandpaper
Measure the size of the loop to get the correct block length.
We draw the layout of the future block on paper.
Redraw it on plywood, (the material from which you make the block), the same width as the emery tape. Cut with a jigsaw along the outline of the drawing.
Grind the cut workpiece, align the edges, bring to mind.
To tension the tape, use a wood insert and move it up or down along the curve by pulling it in or out.
Fast and cheap! Good luck!