Are there T5 fixtures that can sit on the lid?

[A. grandis]

The optimal for a 27" wide tank would be 12 bulbs (2 of 6 bulbs each). Specially if the tank is also 27" tall.
Each of the 6 bulb fixture is 13.4" wide X 2 = 26.8"
You could also go with 10 bulbs (1 of 6 bulbs + 1 of 4 bulbs, total 13.4"+9.4"= 22.8").
The wider they sell for a single unit is 8 bulbs which is only 17.5" wide and because your tank is 27" tall you should have more bulbs than that.
I like to use metal halides for tanks deeper than 24", but it can be done with T5s as long as you place the corals accordingly and with the right amount of bulbs.

 

[Jrapa86]

I would just get an ATI Sunpower that would cover the whole surface area of your tank and call it a great day! You can place the ATI fixture right above the tank resting on pieces of wood. You shouldn't do the same with LED fixtures! A regular Sunpower will grow any coral and will last your life time! The picture below shows one of my 75gal. systems with a dimmable Sunpower. I placed a canopy above that fixture.

How hot does that wood get with the light?