I'm looking for a tool to smooth/flatten down a surface in a rectangular channel in the center of a piece of wood (I constructed the piece by gluing two halves together, but stupidly didn't clean the channel halves well prior to gluing). The channel measures ~1" wide x 2" long x 8" deep (you can think of it as an 8" deep mortise). It's roughly square, but needs a lot of help. Only one surface actually needs to be flat/smooth.
The best tool that comes to mind for the job is a file or rasp, but I need the handle offset so that it can actually get in the channel (the piece's construction is such that without an offset handle I wouldn't be able to put the face of the tool against the surface of the wood). Alternatively if there were a way for me to sand inside the channel that would work too, but I'm unfamiliar with any way to get sand paper in a channel that deep/narrow.
After a ton of searching for things like "crank necked rasp" and "offset handle file" I finally learned that there is a name for what I think is my dream tool: straight ironing rasp. This, for example, looks great:
It's pretty pricey and ships from Europe. There seem to be tons of curved ironing rasps available from many of the usual online stores in the US, but very few straight options. So far this is the only one I'm able to find. This link implies that Auriou used to make a straight one, but now I can't seem to find it anywhere. Am I missing something here? Why are there so few of these tools? Does it go by a more common name that I'm just unaware of?