My Savage .22 has a hollow synthetic stock. It felt way too light in the rear, didsn't sit into the shoulder well.
I decided to put all that space to good use and add some ounces at the same time.
I had a few flat tins that cold medicine came in. I filled one of these with spare ammo, with a closed cell foam spacer between layers to prevent rattling around. I found that by using a wide rubber band around the tin it fit securely into the butt , the rubber band resting on the moldings for the buttplate screws.
There was still space to spare so I filled the rest in with a cord pull through and patch materials, a small tightly capped bottle of balistol, and a small lockblade knife, and a spare magazine.
I normally carry a knife with scewdriver blades in the field, but I figure to replace the philips head buttplate screws with flat slotted screws that could be unscrewed with the edge of a buckle or knife blade if need be.
A rubber gasket, cut from an inner tube, would keep the contents dry.
I may install sturdy snap fasteners in place of the buttplate screws.