There's not much to platforms. Your shop probably only has one brand, and that will be fine - you'll only be on them for a month or two There are some that are designed for downhilling, with spikes to keep your feet from slipping, but I don't think you need anything like that. Just get the basic ones to get you through this time until you're back on the clipless.

Edit! Just read that you have pedals with straps. You can just take the straps off! Easy!