Since a RD-75 dipole is rather tall, a practical solution that comes to mind for woofer placement seems to be sidefiring at the bottom of the dipole baffle. The bottom part for the dipole could be made a monopole with adequate trapping of the backwave.
The graph below shows that this is indeed a feasible solution. A 21 inch wide test baffle with RD75 was temporary converted to a hybrid. The bottom 18" were made monopole. The box, 20" wide, 18" high and 15" deep was secured 8 inch behind the ribbon. The resulting cavity was stuffed with 6" thick open cell foam, the remaining 2 inches with hollow core fiber. No special reason to go this way, just available materials.
The red trace is the dipole at 2 meter without monopole annex. The white plot is with monopole section. No real negative effects visible in this plot. (The microphone was placed 38" high above floor level, the RD75 was sitting 4 inches above floor level.)
Measurement related items: this measurement was performed in a space 40' long by 25 ' wide and 20' high.
The driver is placed equidistant from backwall and sidewalls. All echoes removed for pseudo anechoic measurement. Passive notch filter was used for cavity resonance suppression. Measurements were 16k MLS with 4096 FFT over a 13 millisecond data interval. Smoothing set at 1/12 octave. (Pseudo anechoic measurements don't require much smoothing since all small room interaction echoes are gated out.)