I used 3 perimeters and 20% of infill, but I think 15% is also ok. The M3 threads are modeled so you won't need much effort to put the screws in place.
I printed it in PET because it should be resistant to chemicals, humidity, grease and so on. Also PETG seems like a good option. PLA should also work, but I am not sure how well it would stand the contact with chemicals such as the chain cleaning and stuff like that.
Yes, the print in the pictures came out horrible in terms of aesthetics, but that was because that filament spool I used might have been exposed to high moisture levels. Anyways it didn't compromise the functionality of the cleaner.
About the tootbrushes
I used the flush cutter that came with the Ender 2 to cut the heads of the toothbrushes. It is really easy to do it this way. Just make sure that the length of each toothbrush head is less than 30 mm so that the lid can be closed.
Some toothbrushes have rubber details that may difficult the insertion in the tool. You can probably force them into place, but I don't recommend that because I'll make difficult to extract them once you need to replace them (don't ask how do I know). Use a flush cutter to trim off any protruding artifact from the toothbrush head before inserting it.