Check out rtd-denver.com for more details, but on there they encourage long-term parkers that are using Skyride to park at either Stapleton or 40th/Airport Blvd.
An off-site option would be FastTrack airport parking, formerly PCA. They've cut their prices recently to $6.75/day plus $2 one-time fee. They also have a punch card and after 30 days paid you get a 10-day stay free.
For what you are looking for, I might look on Craigslist and see if you can find somebody located convenient to RTD that has a garage for rent.
When you add it all up (insurance, registration, maintenance, parking) you might be better off renting with Priceline or something.