If you can remember your maths, work out the volume of the cake mixture (ie the area of the cake tin they want multiplied by approximately how high up the cake tin the mixture would fill to) and then see if you've got another cake tin that will take that volume of cake mixture. Admittedly the baking time will be different (and sometimes the oven temperature), but it seems to work remarkably well. I've made quite a few cakes in casserole dishes for this reason anyway.