Gel toppers tend to breathe better providing somewhat cooler sleeping is our experience from the many toppers we have tried over the years. Most toppers should pose no problem if attempting to cut them to size. We have purchased a couple of toppers from Samsclub and they are well made and have performed well.
Keep in mind that many toppers will have a base foam with the memory foam layer just on the last inch or two, thus a 4" memory foam topper may only have a inch or two of actual memory foam. One of the first mods we did was to get a topper, my DW and I had back-aches first time out and was the only negative issue we experienced on our maiden voyage.