A jet ski trailer once modified should make an excellent TI trailer. The trailer I use is actually a 3.9m dinghy trailer, though it’s similar to a jet ski trailer. As Tom said you may have to lengthen the drawbar. For balance it would need to measure more than half of the TI's 18.5' length so around 12' or so (allowing for jockey wheel etc) from coupling to axle.
It’s also just as important to lengthen the rear of the trailer so that the TI’s overhang is properly supported. This will give you a location for mounting the lights/licence plate so a flag won’t be necessary and it will always be legal.
A simple way to do this would be to bolt 2” square tube 10’ long or so directly to the trailer (U-bolts are what I used) and mount the TI cradles to the tube.
These pics will give you some idea. The standard dinghy trailer originally ended just aft of the trailer wheels. I have a very steep driveway to negotiate so I ripped some 4x2 RHS diagonally to give a taper to the rear extension to stop it scraping.
PS- The third TI cradle makes loading/unloading very easy, acts as a rear support for the TI and a tail light protector.