Making a silnylon tent fly recently, I simply used additonal pieces of silnylon laminated at stress points. The result was very satisfactory - much stronger than anything else in the same weight range that I could have imagined; and although I used a roller, I did not use clamps or weights.
The trick, as discussed on a recent thread by the author of the BPL article on laminatng silnylon, is to use the GE silicone GLUE, NOT the sealants, like SilNet. Am going to peel up portions of the reinforcing patches on a recently made prototype tent canopy and reseal the edges with the glue, where SilNet was used before. The glue is awesome, and cheap.
On a different note, have you considered using the 1.26 oz. and 1.5 oz cubens that you can often buy in small quantities from Zpacks? Awfully strong, yet still as light as silnylon and much lighter than the Xpack materials. But yes, the least costly route by far is to laminate the silnylon. There is 2.4 oz silnylon available from Seattle Fabrics if you want something stronger.