Talk about Bubba gunsmithing. Could you at least undo the two screws holding the stock onto the action and work without the contortions and risking damage to your scope and action? LOL. I had one of those in a guide gun and found the claimed spreading out of the recoil force by the moving mercury wasn't much better than simply adding an equal amount of lead weight. I drilled a hole in the wood stock ,filled the extra space with silicone caulk and put the buttstock cap back on. I compared it to sticking in two or three equal diameter lead egg sinkers and didn't see much difference. I never found out if the mercury had to be at slight incline, decline or level with shooting position to get the back and forth action for most recoil reduction. I had mine at a very slight incline so the mercury would flow towards the front when holding gun level. Hope it worked for you.
The mercury tube should be inclined towards the front, so the mercury pools to the REAR of the buttstock when level. The recoil pushes the rifle backwards, the mercury climbs forward and finally hits the front end of the tube... at which point it's basically just a lead weight using inertia to slow the recoil movement. Ideally you have baffles or ridges inside the tube for the mercury to crawl over/through giving you multiple "resistance" points throughout the recoil motion. To work effectively, they need to be several ounces, about 6oz or more total, but also the longer and more baffled the inside the better. The goal is to slow that recoil pulse into stages instead of one solid pop. Many "recoil reduction" systems use fixed inertia increases (just more weight), or increased staged spring rates using multiple-springs with different rates to get a similar "staged" engagement... but those ideally require level mounting, preferably inline with the bore axis... that's not a requirement for mercury systems which can use gravity to increase the resistance when the rifle recoils backwards.
Just drill a hole slightly larger in diameter than the weight in stock and secure it in place with epoxy. You can use a heat gun to help get rid of air bubbles in the epoxy when you pour it in.
