I use something called, plumbers putty to adjust the cg when I get it where I want it to be and am happy with the way the plane fly’s then I make a more permanent adjustment like was said here stick on weight or drill and use screws to attach weights. the plumbers putty is sticky enough to stay put to test an airplane, it however doesn’t work well in cold temperatures as far as staying put. you can stick it in some pretty tight places though.