Ball valve is loose in the UDS

Can you post a pic? That might help us help you.

ETA: Wait. Your ball valve comes out of the bottom of the drum? Try putting an elbow and a 2' piece of straight pipe on it to bring the valve higher.

OR

Tighten your nuts.

Are you using conduit nuts to secure it??

If you cant weld or don't have access to a welder you can do this;
remove the nipple with the valve on it install a conduit nut on one end so that the points on the nut point to the middle of the nipple
screw the nipple in from the inside until the conduit nut contacts the drum
put 2 1" dia flat washers on the nipple on the outside and secure with a conduit nut points facing out gently use a screw driver and a few light blows with a hammer handle to snug it down
reinstall the valve.

The points on the inside nut bite into the drum when the outside nut is tightened. The washers increase the surface area and give the outside nut a flat surface to push against like it is designed to do increasing the holding power.

Tighen you nuts or epoxy putty, maybe silicon caulk on the outside if you can't weld.

jb weld, outside only. It'll stay put.

We really need a picture of how it's installed to give you advice that will work for you.

I had the same problem even with the conduit nuts. JB Weld on the outside fixed it right up.