For Hannakah, I made a magnetic bead stretch bracelet for a friend who decided she wanted a specific clasp added to "dress it up" from the bracelets she normally wears (also made by me). Why make a bracelet elastic and still use a clasp? Well, because she doesn't like anything loose around her wrists, she likes her bracelets snug but not tight and still movable and expandable -- she has carpal tunnel in her wrists and arthritis in her fingers and her wrists and hands sometimes get swollen because of it.
I used a combination of 2 things -- a crimp cover and I now can't remember what they're called specifically, but they're like wire or thread protectors...those little things that you string your wire/thread through before attaching to a clasp or jump ring, to give it more strength and stability, especially when stringing heavy or expensive beads or pearls.
To start, I strung the elastic cord through a protector and through one part of the clasp, back out the other side of the protector and tied a knot, leaving about 1/4 to 1/2 inch tail. I strung the bracelet to length and did basically the same thing at the other end with the other part of the clasp and a second protector and also leaving a tail after tying the knot. I tucked the tails into a few of the beads and trimmed where I needed to. To finish, I used coordinating crimp covers, very carefully closing over the exposed part of the knots on each end and making sure the elastic cord was threaded through the crimp cover and not crimped over by the cover when closing it.
She loves this bracelet so much that it's now a part of her normal daily wear, unusual because she's not a big jewelry person and never wears a watch and only a necklace for special occasions. It's been over a month now and it's still holding strong and pretty. I wear magnetic bracelets everyday too and I'm even thinking of doing one up like this for myself.
Hope this helps. Good luck!
Dawn 
Blessings to all, take care and be safe!
Dawn
"When you're faced with the necessity to do something, that's a stimulus to invention." Arnold O. Beckman, PhD.
"A woman is like a teabag -- only in hot water do you realize how strong she is." Nancy Reagan
My Artfire studio: http://SunriseJewelry.artfire.com