It was not a bag from a known brand. I brought my handcuffs to the bike dealer and had a try to check whether they would fit in their bags or not. You need indeed quite a big bag. If I remember well, the handcuffs dimensions are 18 x 11 x 5 cm.
Note that the straps of a standard (non Strida) saddle bag are too short too attach it to the seat tube: you might have to make them longer.
