I can't think of a way to "fill" that gap with anything that won't "stick" to the bolster(s), that won't look terrible. The way I would do it is to hand sand the angles on the front of the scales, using a granite block (or you can use a piece of glass) with a sheet of sandpaper on it..... take a stroke of two, trying to hold the scale at the correct angle, and they test fit it....keep doing this, and test fitting until you get the best fit you can. AFTER that is done, then drill the screw/attachment holes in the scales.
It looks like you already drilled holes, so doing as I am recommending would require starting over with new scales.
I suspect you're thinking...... "I don't want to have to start over on the scales.".... but keep in mind, when it comes to knifemaking, you can have "good" or you can have "easy".....but rarely can you have both.