Just abolish the damn border and establish a common immigration system. Both Canada and the US have deeply flawed immigration systems, by opening internal borders and switching to a common scheme both countries have a politically less decisive time to renew and update immigration schemes. By working together with a common border we can save significant resources in both countries.
As it stands both Canada and the US allow their border officers to perform inland enforcement actions, by working together we could still police and patrol the border (open does not mean unsecured) for drugs, guns etc. Heck, inspection stations like you see when entering California could be an option at high volume crossings, they exist in the EU too and are opened and closed depending on current events.