Well, since I suggested you try here, let me start with a couple of suggestions. There is a Shuttlepark and a Shuttlepark2. We have used both:

http://www.shuttlepark.com/shuttlepark/

http://www.shuttlepark2.com/shuttlepark2/

The discounted daily rate through the web coupon at SP2 is about as cheap as we've found,and we've never had a hint of a problem at either place. I suppose the suggestion about not parking near an exterior fence is a good one if one is particularly concerned.

Several years ago before we were flying so much we found an Internet site that advertised the cheapest parking I'd seen. The only catch was that you had to prepay and there were no refunds. We did, and when we showed up at this place... well, let's say I'm glad we were driving a big old Ford Crown Vic because we had to leave our keys with them too. They parked it in a kind of hilly field. The car survived but it made me suspicious.

I have the feeling as we wait in the shuttle spot at SEA that other shuttles come around a little more regularly (for example Doug Fox), but worse case, it's no problem just calling these outfits as they also cruise fairly regularly and the longest I've ever waited is about 20 minutes from the time I use the courtesy phone at the pickup area.

I'm also assuming you don't want to include an overnight because of your proximity. Sometimes there are good Park and Fly deals at motels, or even free parking at some of the outlying ones, but any of the ones we've found haven't included really secure parking.

Others with more experience may be able to offer some other alternatives. I'd be really interested to see them as well because we're always looking for something a little better.

Cheers,
Fredd