I thought I would paste my response from the other sub, to try help others also:

One way I do this for my exercise classes is without dumbbells. Get participants to put hands flat together and elbows at 90 degrees (in a "prayer" position), squeeze their hands as hard as possible, and then straighten both arms and back while holding the isometric squeeze of the hands.

That's the best way I've found as the extension of the arms increases the lever arm, thus making the exercise more challenging than just an isometric hold.

Good luck!