The diminished seventh chord (dim7) is symmetric: it repeats the same shape every 3 frets. With just three movable shapes — one for each string the root can fall on — you can play it in any key, anywhere on the neck.
The lowest shape: the root sounds on the 6th string (low E), with the 5th and 1st strings muted.
Same idea, one string up: the root sounds on the 5th string (A), with the 6th and 4th strings muted.
The most compact shape: only two frets wide, with the root on the 4th string (D) and the 6th and 5th strings muted.