Lookout would be a good choice. The parking lot is literally about 10 yards from I-90, and it's likely they will have some snow.
Boise is quite a long way from Glacier. There is no easy direct route. If you did fly to Boise, you'd likely go over Lolo, not Lookout; Lolo could have some snow, too. The drive through Idaho over Lolo is one of America's great wilderness drives; in 150 miles you might pass three gas stations.
If you don't have tickets yet, I'd recommend Spokane as a cheaper place to fly to; 3 hours from Missoula, or you could take a shortcut to Glacier by going through Sandpoint; Sandpoint is highly cute and above it is Schweitzer where there will most likely be some snow. Missoula flights are often expensive; Bozeman may be cheaper, and Helena has the odd cheap deal. Kalispell is generally expensive though the closest to the park; Great Falls is not that much further but on the other side.
Unless you have a compelling reason to go to Boise, the money you'd spend on more expensive tickets could be offset by less gas. Also note that Frontier flies into Spokane and Bozeman.