This is a question about a specific solution for a problem that's been asked before.
The best solution is u-channels that the blinds' hem bar can travel up and down within. This solution can be seen online, but not in any DIY way. A few window treatment companies offer this kind of solution with purchase of their blinds, installed by a local contractor/professional. Examples:
https://www.rollashade.com/side-and-bottom-channels.html
https://www.draperinc.com/windowshades/productdetail/40/side–u–channel,–h–channel,-or–l–angle
https://vertilux.com/en/products/side-channel-profile
https://www.hunterdouglas.com/lightlock
A few DIY solutions exist that are essentially L-channels (rather than U-channels) like this which I may eventually go with, but I'd much rather find a good u-channel solution.
The only DIY solution with a U-channel that I've found is this one from Sweden.
I just find it hard to believe that this fairly simple solution is only available from a vendor in Sweden (who won't ship internationally, I asked).
Does anyone have any ideas of a simple plastic (or metal, but plastic would be easier/preferable) u-channel that could work like the Sweden solution above, and is available in the USA? They would need to be about 1/2 inch wide, by about 1 inch deep.
The closest I've found is a u-channel from McMaster-Carr, but it's only 1/2 inch deep which probably would not be sufficient.
Best Answer
I might try some angle L in plastic or aluminum. I looked on line and there were lots of options I did see 1 MFG that had a few colors. I searched with the term “plastic angle L” the aluminum did show up also.