All of the above advice is right on the money. You are wanting to use the wrong equipment for the wrong reasons. Spoilers do just that, they spoil lift. When spoilers are deployed your wing looses lift. When the wing stops to produce lift it is no longer flying and you will have very little control over the plane. If you couple that with your situation of trying to drop into a runway that is small and has not approaches (because of 30' trees) you are working with a perfect recipe for disaster. Spoilers do not slow down your plane at all. It will continue move at the same speed as before the spoilers deployed, however that speed will not be producing any lift so the plane will be falling out of the sky.

RC gliders use spoilers to simply do what I have said above, make the plane quit flying. They usually are used to "spot land" a glider in a target circle on the ground. When the spoilers deploy (at a couple inches altitude) the glider will simply drop to the ground, but still moving just as fast.

Ken