Can you drive up the North Shore to interview at the Grand Portage border crossing? That GE interview center seems to keep long hours (IIRC something like 8 am to 8 pm every day of the week versus something like 9 am to 4-5 pm Monday through Friday only at MSP) and might have better availability.

There's yet another GE interview place (this one is also NEXUS) in MN at a border crossing in the north central part of the state, accessible only by secondary (and worse?) roads, that offers interviews during more limited hours, maybe only three days a week. You might want to look for an interview slot there too, if it's feasible for you.

Otherwise, there's MKE and ORD, both of which are rumored not to have much availability but at least are drivable, and IIRC someplace in the Dakotas which looks like a long drive at least from Minneapolis.

Beyond this, can you schedule an interview at some other airport you're flying through anyway? That seems to be what many people are doing to get their GE final approval.