I'm sorry I am a little confused? What's with all the talk about screws and bolts? All the thru hulls I've seen are threaded with a large jamb nut on the inside so the only hole is for the pipe going thru the hull. There where a few others using a couple screws to hold it from spinning and keeping it in position. I agree on the 5200. Just my confused 2cents.

On the thru hulls I did, I also coated the the inside of all drilled holes with resin. A little extra measure.

the thru hull is threaded, but the sea cock threads onto it and has 3 holes around it. to me it looked like a few 1" screws would hold it from spinning.

here is the thew hull i will be using.

http://www.westmarine.com/webapp/wcs...0&classNum=217




here is the seacock i will be using. the seacock is threaded to attach to the threw hull.


http://www.westmarine.com/webapp/wcs...70&classNum=71

the seacock pictured is not needed for your use, unless the thru hull is cut to perfection as for length.. the 3 hole flanged type are used when the fit is not so snug and the thru fitting is not too long. the 3 holes are for mounting then installing thru hull that is correct and accurate length from outside with proper tool.
keep in mind the 5200 is very, very strong and in the need for repair or a broken seacock it will be a chore to replace. 4200 is slightly more flexable and is more user friendly when the need to repair arrises.
if this is an area in the bottom you may want to recess the thru hull into the gel slightly by sanding with the proper diameter sanding disc on a grinder or drill. then use the sealant as a fairing compound.

good luck.