Wow! Building at warp speed again, I see... slow down!

Better method of thinning epoxy is to gently heat it. Alcohol will weaken it, and may not cure quite right. I personally would not sand the braces down, this is one of the few areas where it's important to get a good tight fit.

Whatever you do, do NOT glue the dihderal baces in with CA! Use epoxy, heated to run into the joint and clamp it tight overnight. Use some scrap of the same material to practice until it's second nature - it's actually really easy to get right after an attempt or two.