I added 48" onto the 3 rails on the trailer on the front of my picklefork trailer(just cut with saw and added in) for that exact purpose,(Yam 750 waverunner) because we got tired of using 2 cars to go out to the lake for a weekend.
When we did that we also added 2 braces to the wooden runners as we put the boat onto the trailer since we dont pull it up as far(weight slightly more to the rear to offset the additional weight) any longer due to the tongue weight although before the modification the tongue weight was 280# and with the ski it was 550#. I use a class 4 stle hitch that we welded direct to the frame of the SUV since I pull a race-car trailer too.Problem is if ya dont have a wheel on the trailer to wheel the front round it takes 3 men and 2 small boys to move the trailer now or a good dolly!
Most welders will charge you by the job, or about 25 Hrs/hr, cut your metal to length, have some plans drawn so "the welder" isnt guessing, but I'd say materials and time should get ya close to 350.00--Kansas Money anyhow.