Here is my version of it for my 29G. Most of the wood/hardware I already had so it has been very inexpensive. It is setup for 3 lights but I only have two in it now. I have wired the two lights into one plug that will go into a timer.
I drilled holes into a piece of plywood. I had to chisel the plywood to make it thinner so that the shades could fit. The plywood fills the space that the clamp usually attaches to the light. Each light has a little ridge that won't fit through the hole so that is how they won't fall through.
Attached to the plywood is 3" wide pine boards to make it look nicer. I would suggest going with at least 4" boards since you can see the wire sticking out the top of the light.
I then used chain that I bought at Home Depot to hang it from the ceiling. I have some S-hooks in the middle so that it is easy to raise and lower the lights.