Try one of these pictured below, they are made to aid in radius in molds.
It came from Mini Craft of Florida, INC Phone # (800) 282-8244 on page 43 in their catalog. It work really well, much better than a spoons, sockets or other items I have seen used. They are made for installing wax fillet but work well for clay or thickened resin. They sell four different ones with 8 sizes. Each has two balls on each tool. I don't think they would cost much and be well worth the money. I would use the cake icing bag thing you see people use to put the thickened resin in the corners then run this over it and smooth it out giving you a perfect radius. A house hold spoon or a mechanics socket on an extension works well too if you cant get one.
