how do you mount a hacker motor on the great planes flatana?

Which Hacker motor, outrunner or geared?

If it were me, I'd install a 1/8" lite ply mounting plate, cutting it into the foam as appropriate.

Here's how I mounted my Hacker A-20 34S on a flat foamie. 1/8" lite-ply and some balsa tri-stock. Hope this helps.