An easy fix would have been to simply glue the rifle to the hand after painting, but this solution would also limit posing with other weapons. Tying the weapon on using clear fishing line is also an option, but it wasn't aesthetically appealing. The solution I chose was to utilize the 1/100 builders part set of Zeon style hands.
This package of hands comes with two plates of six hands each. On each plate you get 2 weapon holding hands, 2 clenched fists and 2 two open hands. The Sinanju's rifle fits perfectly into the weapon holding hand.
Besides the obvious advantage of being able to securely hold the rifle the hand also looks better with a higher degree of detail than the included jointed hands. If there is a down side to the the builder parts hands, it's that they appear minutely too large for the MS.
I was also able to modify one of the closed fists so that the Sinanju can hold the bazooka in a over or under hand grip. This modification only requires the removal of some of the material creating the clenched fist, the hand can then be glued closed and the handle can be slide into the gap of the hand.
This is by far the best solution I was able to come up with. The hands work right out of the box, and they're cheaper than purchasing the 9 individual parts to create a second fixed set of stock hands.
If you're interested in purchasing these parts to modify your Sinanju consider purchasing them through my Amazon link below. Every purchase helps me host contests and other giveaways.