Dig it out so there is no loose material, at least 3" deep. Frame it with 4x4" PT wood, such the top of the rubber will lie flush (on flat ground) to the earth, build up and retain the low side/s, slope out the high sides, install drainage as required. Make your frame so that your screw can go in 1.75" from the edge of the rubber, and land in the center of the 4x4. Pretty much, you are just building it like you would a concrete pad, with a beefier, permanent form.
If you dig 4" or deeper, fill with 1'"-minus gravel (drain rock OK for wet locations, or crushed) to 3" deep. Tamp with hand or power tamper. Then fill with 1/4" minus crushed gravel.
Tamp again thoroughly with hand or power tamper.
Crown the tee slightly so water will drain off. Pay attention to the corners. Re-tamp. Remember, no matter how well you build it; it will sink, so crown it up plenty. I will settle out, and you don't want low spots to form; they'll collect water, even with "porous" rubber.
Slip a fender washer onto the a screw, coat under the washer with with goo (below), and screw down a corner. Use 3" long, strong screws, stainless if you can afford them. The bigger the fender washer (stainless, if you can afford it) you can find, within reason, say up to 2", the better they will hold down the rubber and resist tearing. Install rubber, screw down like lug-nuts: criss-cross, and don't tighten them all until all the screws are started into the wood. Screw each corner, and at least every foot along all 4 edges. I like to coat the contact side of the fender washer with Marine Goop or you favorite amazing solvent-based Shoe-Goo type adhesive, for further reinforcement of the screw points.
Go ahead and tighten the screws until the fender washers suck down in to the rubber. We will tolerate these small, sealed low spots as opposed to having the washers high enough to slip on. I like to install them so that your shoe won't touch the washer, but will get a slight bit of traction on the screw head, without catching on it.
You can patch broken tees, too. First you need to fix the torn-up gravel. Tamp it back, adding new gravel as needed, to match the original crowned shape. Re-tamp. Then, clean up edges of the tear with solvent cleaner, and glue with solvent adhesive. Keep off tee for 24 hours. Cut out heavily damaged areas, trim up a replacement "patch" of spare rubber, clean edges, and glue into place.
Sorry for the unstructured reply, just streamed it out there, let me know if I can clarify anything.