You can do it yourself. The Criminal Code covers what needs to be done. It doesn't apply to Lee-Enfield mags.
"a) the indentation of its casing by forging, casting, swaging or impressing;
(b) in the case of a cartridge magazine with a steel or aluminum casing, the insertion and attachment of a plug, sleeve, rod, pin, flange or similar device, made of steel or aluminum, as the case may be, or of a similar material, to the inner surface of its casing by welding, brazing or any other similar method; or
(c) in the case of a cartridge magazine with a casing made of a material other than steel or aluminum, the attachment of a plug, sleeve, rod, pin, flange or similar device, made of steel or of a material similar to that of the magazine casing, to the inner surface of its casing by welding, brazing or any other similar method or by applying a permanent adhesive substance, such as a cement or an epoxy or other glue."