The risers are just 1" diameter aluminum drilled out 1/2" in center, angle cut to match tower drop, and a 1" weather-resistant EPDM rubber sealing washer (-20 to 212 deg. rated) on either side. I have a 2" long stainless steel 1/2" diameter bolt that passes through the rack, washers & risers into a 1/2" tap on the tower. You can see the "button" head of the bolt in at least 1 of the pics - allen head indention. I also had to drill out the rack slightly to pass the 1/2" bolt, but the drill out will also prevent any sliding of the rack which I wanted.
I checked the cross bar clearance like 4 time before I tapped the tower. Nothing like a little pucker factor drilling 4 1/2" holes in a perfectly good tower! I too ended up going wider than planned. The tow rope won't be an issue for surfing. I expect I might have an issue wake boarding with boards up there, but I mostly surf & made the compromise. If I went out another 4" I probably could have completely resolved the wake boarding issue, but the tower really starts dropping off fast on that part of the tower.
I doubt you'll have an issue with the tow rope in your setup given the height of your mounts. Definitely like the functionality you put into yours. I was trying to make mine as low profile as possible, but I had to take the tow rope limitation as a potential trade off to do it.