Try lacing the top at tight as possible,so you can only fit three fingers between your shin and the boat,the less room the better.The buckles with the lace system should be able to get tighter, you may have to work it out.
All full tilts are bodyheat molded, the padding may be pretty worn.Try some basic foam in the shin area,from like a camping pad until you aren't sore and then try tightening them.
When in doubt check out the local boot fitter, they may have a method.