Just use a dremel. You wanna open up the button holes like 1 - 2 mm. Just open it little by little until the button fits. Screw type might work better but snapins are good too, just don't open them too much or snapins might fit loose.
If it is in good condition you should consider selling the Agetec stick you own and buy any other stick (for the enclosure) plus encoder (or do a padhack for DC), Agetecs have gone up in price quite some.