As long as the bolts clear any adjacent components, the orientation doesn't matter. I normally place the bolt head on the coupler and the nut and washer on the flex disk side. You definitely want flat washers where the nut bears on the flex disk, but no need for lock washers if you have nylock nuts (which you really should use here).
Also, as mentioned, a second flex disk might fit here as the one disk is flexing quite a bit to take up the space. Not critical, but I'd run two.
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