Best way to motivate an SDR team is to tell them that the number of Spiffs you obtain over the course of the quarter will be taken into considerations for promotions. I know when I was an SDR, if you told me I could do XYZ to get promoted faster, I would do it in a heartbeat. At the end of the day, no one wants to be an SDR, so anything you can offer to get out of that, I imagine they would be into it.
Or, offer WFH days as the spiff. If they hit their goal or the spiff, they can work the remainder of the week from home. Then they can decide if they want to use that time at home as a "day off", or if they will continue to work (or come into the office), they have proven themselves as a good candidate for promotion.