1/4 is a little large. Either 10-32 or 10-24 will be fine. In my area, there are more sources of 10-24 than 10-32. Which ever size you pick, stick with it for all fasteners so you won't have any mixing and matching problems. A lot of accessory geegaws are attached with #10.

I also put a #8 rubber washer on the inside under the flat washer to seal the threads. Before putting the inside wachers on, scrape the area to make it smooth and flat. Tighten until the rubber washer is squeezed enough that its OD matches the flat washer.

for nuts, get the nylon insert locknut type.