Mario, the column and cabinet work looks fantastic, really top notch!!
I also used a 1/8 masonite covered with GOM as inserts on the bottom of my soffits.
Although I didnt cut out for lights I had cutouts for HVAC diffusers.
When the masonite inserts were ready, I cut the piece of GOM oversize, used thumb tacks to "lightly" stretch and hold the GOM on a piece of ply. After spraying the the GOM and the panel, I pressed the panel on the GOM.
Now with the GOM attached to the front of the panel and looking at the back of the panel, I Trimmed the GOM ~ 1" oversize around the panel, And cut an "X" in the diffuser opening, that let me fold the GOM around to the backside and attach using the 3M again.
To mount the panel, I applied thin beads of Construction adhesive around the back of the panel. Then used a couple of the "Helping hand" type jacks and 2x4s to distribute the force and attached the panel to the bottom of the soffit.
When trying to cut out the GOM for mounting my sconces, I used staples and had trouble with fraying if I got to close, fortunately my sconce mount was large enough to cover everything.
Dont know if you can get away with the extra thickness from wrapping the GOM around the back side, it will depend on how your trimming it out. But this worked well for me.