Wilderness Exchange is within walking distance of Union and they require you leave a bag with them when you go into the store, they may be willing to stash your bag for a few hours. No guarantee on that, but an option if you can't find anything else. They are only open 10am-6pm on Sundays, so you may have to wait around a bit or pick it up early depending on when you get there and will be leaving.
Number of locking points: Locker doors may lock with either single- or three-point locking, but this is not normally chosen as a separate option, and the choice is usually dependent on the number of tiers in the lockers, or whether they are a high-security model, although some manufacturers do allow purchasers to specifically choose an option here that goes against their normal practice. Single-point locking locks the door at only the point where the latch engages with the door-frame, whereas three-point locking uses extensible steel rods to lock the top and bottom of the door as well.
Sloping tops: while most lockers have flat tops, some manufacturers offer the option of sloping tops to their range of lockers. The slope may be of either 30 degrees or 45 degrees to the horizontal, sloping towards the front, and the purpose of this is to make it impossible to store items on top of the lockers, or to make it harder for dust or other debris to accumulate there. This is an important factor in places like food-processing factories or restaurants where hygiene requirements must be met.
Stored my luggage for several hours at Lugden. Great location just across the street from Union Station. Spoke with the owner and he said he owns the parking spot so they are always there during the day. Prices were reasonable and I felt comfortable storing my bags there while I explored Denver. The owner was nice and gave me coupons for some local businesses.
The storage lockers were a “pilot,” the kind of small test that city government frequently uses to test a new or controversial idea. The city offered up the lockers for individuals to use for months-long stretches. At the time, city officials warned that “misuse of the lockers, vandalism, or other unanticipated results,” could force them to cancel the project.
Perforated lockers are similar to the standard types of locker, but the door and walls are made largely or entirely of perforated steel, with hundreds of holes creating a strong mesh arranged in a diagonal pattern. This is used where good ventilation is required, or where, for security reasons, it is necessary that the contents can be examined visually while the doors are locked.
With the same standard features as the Traditional line, the Traditional Plus Collection is intended for more rugged applications. Available in single-, double- or triple-tier configurations, this collection features 14 gauge doors with 16 gauge full-height reinforcement channels, frame-secured 11 gauge frame hooks with protruding padlock hasps, and a maintenance-free, single-point latching system.
Just so you know I have tried to call and email and left multiple messages trying to find some middle-ground and give him an opportunity to make amends as I know how determinantal negative online reviews can be for a business but they have not returned a single call or email and know that we are back in FL and she has lost/had stolen many, many expensive and sentimental things we have no choice but to share our story so that know on hopefully ends up having a similar experience.  

Locker Denver