Shit happens
Some CGuE Deep Arms suffer the same problem with the beam swords, so you're not the only one I'm lucky that my Destiny doesn't.

Anyway, easiest thing to do here is to snip the pegs of the beam blade. Don't cut the whole thing off, just trim it till the blade fits into the socket nicely. If it still doesn't fit, slowly scrape and file the end parts of the blade till it is small enough for you to shove the blade in. Some glue might be required to attach the blade firmly after you trimmed it.

If still cannot, well like what chris said, mod the sword... Not easy to do if you don't have the tools.