A 2x54W kit installed directly to the top of your canopy can be arranged as in the photo below.  The white board is 11.25"W and 48"L so you can see that even in a canopy over a 12-13" wide tank you could fit as many as four 54W units since each reflector is 2.5" wide and the ballast is only 1.25" wide.  The minimum mounting length, including space for the sockets and wiring, is 47.75".

A H Supply's 54W kits also include the wire handling hardware you need to keep things neat, including the mountable wire tie holding the bundle of excess wire just to the right of the ballast.  In fact, except for the board and the cloth background, every single item shown in the photo above is included with the kit.  Other kits we've looked at always seem to leave out parts that will obviously be needed.  One even neglected to include the extra wire needed in addition to the ballast wires to complete the wiring hookup. At least that problem was solvable, even though #18 solid wire is not easy to locate. There was no fix for the mill-finish reflectors we found, or for the unpotted indoor-type ballasts that shouldn't be used anywhere around water.

If your canopy has a top that flips up, there's a disadvantage to mounting your lights (or, at least, all of your lights) to the top.  Your tank will not be well lit when you need to work in the water.  The solution is to suspend your lights within the canopy.  One way to do this is to make a fairly simple framework out of 1x2 boards as illustrated in the photo below.

This framework was built for a 1x54W kit plus 2x54W kit.  It uses three 4' long 1x2 boards with shorter pieces of 1x2 board connecting them near the ends.  Leaving just 1.5" between the long 1x2s allows plenty of space for the 2.5"W reflectors to be mounted on the bottom side.  The ballasts and power cord are mounted on the top side and we drilled holes near the cross pieces to neatly route the ballast wiring down to the sockets.  The framework can then sit on brackets mounted to the sides of the canopy as illustrated in the two photos below.

The brackets we used here were sold in the hardware store as "1 1/2" corner braces."  Since your framework can rest on the brackets securely, it doesn't actually need to be attached to them.  That's handy because it means that you can unplug the lights and lift them right out of the canopy.

Even if you are using just a 1x54W setup, you can still hang it from brackets as shown in the photo below.

And here's what the end of the bottom side of that 1x54W setup will look like if you route your wiring down through a hole in the 1x2.

The 3x54W framework will still leave you easy tank access on an 18"W tank. A 1x54W or 2x54W setup like this should still allow you pretty easy tank access even on a tank only 12"-13" wide.  But with a larger setup like the 3x54w (1x54W plus 2x54W) on a 12"-13" wide tank, you're still going to have the problem of keeping light shining into the tank when you want to work in it. 

One solution is to mount your 2x54W to the top that opens and then put the 1x54W in a fixed suspended position as below.

Here we've used the same type of bracket but secured the long 1x2 board directly to it. Here's a side view.

If you don't want any of your lights mounted on the top of the canopy, a "fancier" solution is to put one light in a fixed position as above, and than hinge your framework for the other lights so they can be tilted up out of the way.  This just involves two hinges, 1x2 cross pieces long enough to reach the back,  and two small blocks of 1x2 board mounted to the back side of the canopy as in the photos below.

If you have a canopy that has a front that opens instead of a top that flips up, you can still mount your lights in a  way that leaves them easy to move.  Go to the deck building supply area in your hardware store and pick up some "3" Angles" (enough to cover the length of the inside sides of your canopy) and some "Rigid Ties" as shown in the photo below.

A few of the 3" angles, shown on the right, will be mounted to the inside sides of your canopy in a line from front to back.  Then you attach the rigid ties (shown on the left) to individual 1x2 boards or to a framework of boards, so the top part of the rigid tie will slide over the part of the angle that sticks out from the side. This will allow you to slide the lights into and out of the canopy.  The photo below illustrates the general principle.

While this survey is by no means exhaustive, we hope it has given you some idea of the options available, or perhaps even inspired you to create some of your own.