An idea for your already-modified silk boxers - If I'm understanding this right, you've cut out the part the bifurcates them and it is now, for all intents and purposes, a short silk skirt. When you spread your legs to sit down, you are increasing your circumference and the silk has no devices to allow it to spread to that circumference (for instance, a kilt has pleats for this purpose), so it rides up in order to avoid ripping.

Try cutting straight down the middle of each side, so that when you spread your legs to sit down, your legs part the silk and you in essence get a "front apron" and a "rear apron" kind of deal. The silk front apron should fall between your legs when you spread them.

I haven't done this myself, so I don't know if it is a definite solution, but I got the idea from how women's skirts have a small slit on each side to fix a similar problem (the slits on their skirts are small because the circumference of the garment doesn't need to widen too much, but when you shorten the garment, and the fact that you would be spreading your legs quite wide when sitting down, the slit needs to be larger to account for this).