It's interesting you had to fab a holder for that unit. On the K75, they have a special housing for it # 61 35 1 459 782 that is located on the fiche for the seat on that model, snaps in where the original tool tray was above the battery. Looked for your bike, but it wasn't located on the same spot in the fiche, maybe somewhere else showing the tool tray under the seat.
did some checking on real OEM seems that your bike does not use the holder from previous models. These are the models that used this carrier for the low seat option.
K589 (K100, RS, RT, LT) (05/1982 — 10/1991)
K569 (K 75, K 75 C, K 75 S, K 75 RT) (12/1984 — 06/2005)
gg