Stainless stick electrodes would do that job better then MIG with dissimilar thicknesses imo. If the fibreglass was in direct contact with underside of the plate where the welding is being done I'd either remove the stem for welding or just bolt the roller on. OTOH if its a distance away, cooling
in the appropriate places while welding (e.g. using wet rags) will stop the heat transferring to the glass. Fortunately, stainless steel
is a poor conductor of heat (relatively speaking) so this is a practical solution as the "red" heat won't radiate too far from the welding area.
Of course, make sure the stem is actually stainless - the half inch thickness sounds a bit suss - (do a scratch test if not sure), because if it's aluminium......