How to draw three slot antenna in HFSS

Hi everyone,
I am designing waveguide fed three slots antenna in Narrow-wall by HFSS. I took box to design waveguide and then tried to subtract the slots from Box but HFSS did not allow this.
Does anyone have the experience of designing a 3 slot antenna in HFSS and you can help me?
The 3 slotted waveguide dimensions which are as follows:
1- waveguide length = 60 cm, waveguide width =9.5 cm
waveguide height =4.5 cm, the waveguide is for operation at 2.45 GHz. is made from Aluminum.
2.slots length = 6.1 cm,slots width =0.6 cm, Slot spacing center to center for other slot=16 cm. end space = 4 cm
please find attached

Thanks in advance.

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