You may be able to retrofit a waist strap on your unit with the straps off an old backpack or buy a replacement one from a backpack manufacturer like Kelty. I actually have one in the garage that I used for attaching a sled to myself for cross country skiing with my kids in tow. It just attaches with clevis pins through the grommets in the strap. Or you could just go to a fabric store and get some 2 inch webbing and some fastex clips and do some sewing. I'm sure there's a solution to the problem. Even just adding a chest (sternum) strap may help to keep the straps in position and transfer some of the weight to your chest and the middle of your back.
What would probably work the best is actually a tumpline. You'd feel ridiculous wearing it but they are very effective and actually pretty comfortable. For those of you not familiar, a tumpline is basically a head strap that you attack to your pack, or in this case blower, that effectively transfers the weight from your shoulders to your back. Admittedly you'd have to be a certain type of individual to allow yourself to be seen wearing one on a customer's property.