The rope loops are almost solid by design, no matter what infill you use.
The section for attachment to the spar used cross infill to increase flexibility.
Going to 100% solid wouldn't really provide any necessary strength, and almost doubles print time... unless you're planning on using them...
Printing orientation was like the far right in photo.
Sliced in Cura, printed on Ultimaker 3 extended.
20% infill, Cross infill pattern
Support on, line type support, 10% support infill
Mine are printed in PLA, which won't hold up long term due to UV/moisture degradation...
It's an extra .10 if you have a local hardware store with a good selection of O-rings sold in singles... free if you have an O-ring sitting around like I did.
...but you're probably looking at $5-$10 if you have to order it.
The thing is - even without the O-ring you could submerge the drain and water will come through the bolt threads at about an ounce every few hours.
Under normal sailing conditions - the O-ring does basically nothing, because the bolt by itself will stop 99.99% of the water.
Couple loops on the underside of your fiberglass piece at the front & rear of the cokpit.
Bungies tied to the loops with hooks/clips on the other end.
Clip the hook/clips to hiking strap attachment points.
The thread is 1/2"-13tpi.
I use a 1/2"-13 x 1" nylon hex bolt. I have a bunch extra and would offer to ship you one, but shipping would cost more than you could just buy one for.
While I personally hate tarps because they are a pain and don't work all that well, so I don't use them as boat covers...
I can get 8'x16' tarps for $10.
$200 is 20 tarps. 40 years at a couple years each... and that ignores the future value of money.
The De Persia bailer plug is 7/8" - 14 UNF.
If trying to us a PVC plug - you would go with 1/2".
7/8"-14 has a major diameter of 0.875"
1/2" NPT has a major diameter 0f 0.84" - and it's tapered, so it should work.
Another option (if you can find any) are SAE O-Ring port caps.
3D print a negative half section about an inch wide and use it as a template.
Here is a quick thrown together negative half of a NACA 0009 if you have access to a 3D printer:
You'll want a symmetric foil so... a common one for this would be a NACA 0012.
Problem is - a NACA 0012 is too wide at a 9.5" chord to fit in the daggerboard slot.
A NACA 0009 will fit, or a NACA 0012 with the thickness chopped down 27/32"