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The best I was able to do was support set to "Touching Buildplate." Support wasn't necessary for vertical screw holes or the bearing holder.
If you want a cleaner surface where the support is located you could try "enable support roof," but I would increase the Z support distance to about .18mm if you're printing in PETG or ABS. Roofs can get difficult to remove.
The 20mm slots for the extrusions are small enough to be bridged without supports. If the part wasn't going to hold the alignment of a bearing I might have tried it, but I decided against it.