lookin at my 30, which is similar in width to your 55 (13 inches), I would not be able to fit a pair of shoplights on top of it, without removing the HOB
filters from the back.
as a solution for this, I took a single shop light and rewired it to be 'overdriven'... so each tube is receiving approximately 64 watts from an electronic 128 watt ballast.
also, this is a high efficiency electronic, so it is good at converting energy into light... most of your 'bargin priced' shop lights contain either a basic magnetic or inefficient electronic ballast; up to 50% of the energy they draw is wasted... the wattage may read 160 watts on a pair of shoplights, but you'll hardly see that as output.
your best bet would be to buy some of those $5 shop lights if you see them anywhere, along with an electronic ballast from GE or Sylvania. saving the sockets and wiring from the shop lights, discard the fixture itself and build the sockets directly into your canopy wiring them up to a single or a pair of electronic ballasts.