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Don't use construction adhesive, 99.99999% of the time it will eventually (sometimes days) bleed through the fiberglass and leave a very visible stain that is not removable.
Like you mentioned lay a bed of mortar and when you install the base apply pressure and wiggle it in a little, also good to lay on the base something heavier until it dries, I often use a box of tiles....something like that. It doesn't need to be a thick base use about as much as you would to lay floor tile (a 1/4" notched trowel worth) unless you are also using it to help level out the base then use it thicker where needed naturally.
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