Sorry about the mix and match for the pictures but I used what I had.
This picture shows how the ski tow mounts thru the engine cover on a '80 CVX 18. On my '86 CVX 18 it mounts the same way. The tow is a quarter turn twist lock so it's twist, pull out than open cowl. No problem.
Here is the topside engine cover (humped cowl as you say) showing where the ski tow comes thru on my '86 CVX 18
This shows the ski tow in it's mounting bracket.
Here's how I done mine.
I found a tow eye on ebay that had a pin to secure it instead of the twist lock, the OD also was smaller than the ID of my bracket so I sent it to a friend (Dick) for some modification.
Here it is as I bought it.
Dick turned a piece of round stainless to the proper size, drilled out the center to match the OD of the pole than notched the bottom to fit the twist lock.
Here's the finished product.
Here are the dimensions for the notch.
Thanks to Brandon (75starflite) for this info.
Here are the dimensions taken from my bracket. The distance end to end may vary depending on the distance between your flotation boxes.
This bracket came from my '80 CVX18 and measures 38" across. The bracket on my '86 CVX18 measures 37".
Here are the bracket views
This is the underside view on my '86 showing the cut out on the bottom front edge of the engine cover
The bracket mounts to the rear seat back on all CVX's. This is on a '80 CVX 18 and seems to be an aftermarket or dealer install but it mounts basically the same as my '86.
Hope this helps.
John