Just my 2 cents, but one should not have to weld extra stuff on a Spratley just to be able to tow it. If anything, I would mod the trailer, not the boat.
The typical bow rollers may not fit that well on a flat bottom jon, but there are accessories for trailers that provide flat bow support for jons (you can google it for ideas). Once you have that, use the front and rear handles to secure prevent sliding forward/back, and strap it down (not too tight) at a couple points across the gunnels. If it's properly positioned on the bunks, it doesnt take much to secure it well.
I have done some research and a lot of people complain about the trailer damaging the bow with the front of the bow hitting the trailer on bumps.
Not sure whats going on here but it sounds like a combo of 1) no bow support, 2) poorly secured, 3) going too fast.