purchased new thicker booties that didnt fit in my foot pockets, no worries...
One esky, 15l of boiling water or near enough, foot pockets submerged in for 5mins, one pair of medium socks, booties over the top, take out and put your foot pockets on, kick back for a couple of mins, cold water over the rubber while your feet are still in the pocket. Done, comfy custom molded fit. You might need to do this a couple of times to get a more comfortable fit. Omer stingrays here.
If you have any particular hot spots or pressure points, fold layers of paper towel and tape that across those areas on your foot before putting the sock/bootie on. I've used this for molding ski boot liners and it comes up a treat.