I bought some blinds that were made to size and my measurement seems to be off by 1/8". The blinds themselves fit - albeit just barely - but with the mounting hardware they are too wide by about 1/8". Any ideas for inside mounting the blinds still but using mounting hardware that would attach at the top instead of the sides? They are fairly big and heavy horizontal blinds, 70" wide and 2.5" depth.
You've asked an XY question, so I'll answer what you should've asked, which is how to resolve the problem so you can mount the blinds as intended.
You almost certainly have 1/8" of extra channel at one end or the other. Use a metal shears or hacksaw to shorten them that much, then mount per the instructions.
Protip: Install felt pads at the ends of the lower rails. They'll make operation smoother and prevent banging from wind, too.