Thanks, Ted. You're right...for a one-time use the extrusions would be overkill. But I can see how you'll be able to get your money's worth due to all of the other options that rig offers. Nice bit of engineering on your part.
Vaughn, I built an assembly with MDF to flatten the slab of Michigan chocolate I got from the Ambassador. It's from a design on the Infinity site. I tweaked the dimensions to suit the size of my slab. Here's the thread I have on the process - there's a link to the Infinity page in the thread.
Thanks, Bill. I made something similar (but on a smaller scale) to flatten some end grain cutting boards years ago. I used dimensional lumber for the sides and ends of the box, and some birch ply I had laying around for the carriage. I just need to make space to do it (either on my driveway slab or on top of my table saw) and get on with the process.
If you wanted to invest in something reusable for rails, the strut material from the big boxes is interesting. Seems like it might work for rails and then come in handy for other shop uses. There are even roller systems available, although not sure they are compatible with the HD material.