The Well MB-06 or ASG Urban Sniper is very easy to modify. This is a sliding stock for it.
It needs two 8mm steel rods (easily available from ebay) and two M3 screws, one of that should be cap screw. The other doesn't matter.
Cut a groove in the steel rod where the lock will hold it.
Screw the cap screw into the lever. It should not protrude on the other side. This will lock the steel rod. Remove it. Insert the lever into the base and put the other M3 screw through it at the bottom and insert the cap screw again. Glue the steel rods in the buttStock .
If you find the buttStock slippery print the buttEnd and glue it on.
I printed it in PLA. I have included the base with inbuilt support. Best to print that without other support enabled.