I solved two problems with a little fix. I use a Stimer with volume on a number of guitars. And on every one, it always slips during gigs. I've used double sided tape and Shertler Putty to try and keep it from moving.....to no avail. The putty works initially, but warms up and slips.
I also have a Dell Arte Robin Nolan import with extended fretboard. There's no way to clip it into the upper, 12 oclock position on the petite bouche.
I know you can flip the pick-up and clip it on the bottom side, 6 oclock side, but the chord is a nusciance and there's still the sliping issue.
I went to a home depot and got a few paint stirrer sticks. I cut one that was maybe 3/4 of an inch longer than the width of each side of the petite bouche, just inside the braces. Then I took a half round file and made a groove perpindicular to the length, about 1/3 deep. Then I could position the paint stirrer, under the soundboard, between the braces, with one hand and slip the Stimer clip under the groove and onto the stirrer. No slips! The pick-up is at the midpoint of the petite bouche, but doesn't alter the tone perceptably. And after a dozen gigs, it doesn't move.