Pre-made grow canopies are farrrrr too expensive for what you get.

Making a grow tent in a garage is a piece o' pie.

Buy a roll of 3m (9ft) wide pandafilm. On the garage floor, construct a rectangle out of 2x4 lumber the same size as the intended floorspace.

Size the tent to fit the size of whatever watering system you intend to use so the 'walls' are as close to the plants as possible, maximising reflection of otherwise wasted light back to the plants.

Put a couple of crossmembers to support a pandafilm 'ceiling' and to give convenient mounting points for lights.

Throw a layer of pandafilm over the rectangular frame to create a 'ceiling'. Get a good friend or two to hold the frame and ceiling up to the garage roof joists while you screw it in place.

Create hanging curtain walls by wrapping the frame with pandafilm. Create a light-trapped entry flap by doubling over the last 3-4 feet of the tent wrapping.

Done!