there are lots of ways to go:
i use large sheets of bark and wedge them in between the glass soemtimes.
i have also affixed suction cups to various pieces of wood and used them to stick the wood to the side of the tank.
In one tank i have a large (3 foot long) cork bark hollow which floats freely (the turtles can get under, on top, and in this thing).
You might also try those foam rubber basking spots that pet shops sell (these come with suction cups attached they do not have abrasive surfaces).
Then there's the "upside down plastic storage" box idea, which as the name say just involves finding one which is just a tad wider than your tank (a 55 is about 12 inches wide on the inside)and wedging it in.
You might also consider a filled plastic stirage box of greater size. Fill it with stones or soil or large smooth gravel and keep the water level just below it. The draw back with this one is that you loose the space under the basking site.
I have also used large tree branches and logs from outside (sterilized in oven first) which were sufficiently big that they could be angled so as to be anchord in the gravel and still stick out of the water.
At garden stores you can get circular pieces of cork bark used to support potted plants indoors. These could be fashioned into basking spots with some ingenuity (and maybe the aforementioned suction cups)
I once built a basking site but drilling holes in several pieces of platic garden edging and some plastic storage box TOPS. I used screws and nuts to hold them togther.
I had a friend you siliconed in a piece of plexiglass at a right angle to the wall of his tank. She used some small wooden dowls to give it extra support. It enede up looking like a shelf in her tank.
Go to a home improvemnet store if there is one nearby and just wander around. You will see all sorts of possibilities.