Adding weight to daily pack

[jason rhoads]

I work all over my city and generally walk to where I'll be working. I travel light for work (nexus 9, keyboard, journal for notes & osprey digistow for misc) using a Russi Stryker 25 for my everyday pack. 

Question: I want to add some weight (5-15lbs) so weekend day hikes feel like a walk with a pack (Russi is generally my dayhike pack). There is a bladder sleeve in the Russi. I'd like to find a 'sheet' of something that can slip in the bladder sleeve that would be adjustable for weight depending on what I carry that day.

Any thoughts?