Something you guys might consider, is using a lighweight, bushed 25.

I have an FP 15 myself and think it's a great running little engine.
But if I recall correctly, it wieghs 6.0 oz with muffler, and my old (Taiwan made, not Chinese) Magnum GP 25's all wiegh 8.6 oz. with muffler. That's only a 1.6 oz wieght gain, but your getting at least a 20 oz. thrust increase.

There's also some simple, low or no cost mods you can do to this engine to gain at least 1000 RPM on a 10x4 prop. This results in at least a 28oz thrust increase over the 15.

I had one years ago on a 2 1/2 lb. foam Sure Flight .15 size Cub, and the thing would take off like a dragster in 5 ft. and shoot straight up outta side. Of course, a cub wouldn't do 3D well.

The bushed Norvel 25 wieghs the same, and makes about the same power as the hopped-up Magnum 25's that I have. I think the OS FP 25 would be about the same as the stock Magnum, and you could hop up the OS in the same way.

I also flew the FP15, and verticle was out of the question. My estimation is that for verticle or attempting hovering, a plane with an FP15 propably cannot weigh more than 24 oz, or about 1 1/2 lbs. Is that acheivable?

But the 25 should hover at least 2 1/2 lbs.

Please let us know how your mini laser 3D works out with the FP15.