I don't have a Trailex of any kind yet looking at pictures of them leaves me wondering if maybe the tongue extension on the 350-S might be longer than that part of your 250? The 350's extra 3+' length (@18'-6" over the 250's 15') has to come from somewhere...

You might try querying the Trailex folks for what they can add about safely adding length to what you have now; the answer might be simply a different tongue!

They engineer their products to be both up to the tasks they're used for as well as safe to use to perform those tasks. Cobbling on an extension isn't something to be attempted without acknowledging the chance that it could easily be done wrong.

An easier approach would be to add a tongue jack with a wheel, and get a short length of 1/2" three strand line.  When you get near the water, tie the line between the SUV and trailer, then disconnect the hitch and roll the trailer back further into the water on the tongue jack wheel.  In marinas with a low slope ramp, this is actually a pretty common way to launch fixed keel trailer sailors.  I have done it a number of times with a 3500# Hunter 23.  Sound be very much easier with a NED because it is so very light.

I like your idea Mark, easier & likely a lot less $$ than a longer tongue. Thanks!

A couple of years ago I contacted Trailex about getting/replacing the main beam on my SUT-250-M2, as I had changed kayaks to one that was about 4' longer and I wanted to reduce the overhang of the kayak to the lights.  I was told by trailex that they would not sell anything to do it.  It looks like the 350 is a different shaped extrusion so bolting on a 350 pc on to the 250 wouldn't work.

This won't add much "length", but I bought an extra long drawbar (ball mount) for the receiver, This added maybe a foot so that I could slide the hull forward for better weight  distribution.

Any way you could install a wider roller(s) to the back end of the trailer, and maybe some more along the length? That way you could roll off and not float off. I try to minimize how much of the trailer gets submerged. Especially in salt water.