Mounting a Bass Control knob
 

I used a step up drill bit, the one that looks like a cone that starts small then steps up to one size larger. measure the blank to find the center and ur gonna make the hole then use the bit to slowly go up one size till it's perfect. go slow to make sure u stay center till its size of the knob then slowly drill a little more to it just barely fits with some spacing around..(this sounds like porn haha)
I couldn't have done it without a dremmel tho cuz ur going to have to hollow out the inside of the blank button (I used the small sand paper bit, a stone bit and a metal cutting bit) all u need to leave are the front and back of the button walls. ur going to dremmel out the sides. be careful removing the button cuz if u crack or snap the tabs it won't stay in right. that's probably what happen with urs.

then the most important part is to use duck tape to get the knob assembly to sit perfectly inside of the button housing. use a lot of tape wrapped around the button then test fit the knob to make sure that when its on the metal rod it sits center to the hole u drilled. leave one small hole in the duck tape wrapping.

now once its perfect I used a rubber epoxy/cement that usually comes in a syringe with 2 sides that u mix, almost like jb weld but it's a quick 15min dry rubber cement from the auto store and I mixed it up and used a small peice of cardboard or anything u can to kinda shovel in the epoxy to that small opening u left in the duck tape. load it up then tape over the hole.

then make sure u use something to keep the hole in the top of the button upright so it doesn't leak out the top.

I waited over night then pealed the tape off.

it should be good.

everything I used is in this picture:
http://images.tapatalk-cdn.com/15/06...001c4b10be.jpg
after
http://images.tapatalk-cdn.com/15/06...82ac1c1efe.jpghttp://images.tapatalk-cdn.com/15/06...89b729ff0e.jpg
what the bottom looks like
http://images.tapatalk-cdn.com/15/06...590d439d20.jpg

Some knobs have a set screw that you can tighten down on the shaft.

Some shafts have a split where the knob slides on. You can spread the "tines" a little bit for a tighter fit.

If neither of those options are available, try some RTV or "soft" Loktite.

Yeah the one the OP has looks like you could just take apart the "box" and just use the knob. drill a 1/4" hole, roughly the blank plate and bolt it in.