I concur with Randy. Often I will make a 'special' sizer for a particular horn. I try to make my sizers about 2 inches in length as they will cover most horns. However, shorter ones will be made as the need arises.
I recall at least 2 occasions where I had nice looking horns, but they were a tad too oval. The solution for both were the same. I measure the inside diameter of where I wanted to make the horn round and made 2 small sizers about 1 1/2 inches in lenght, drilled a hole down through the center and inserted a stainless eye bolt with a nut on the other end. After sufficient time in boiling water to ensure the last 5 or so inches were pliable enough I inserted the small sizer then followed up with a slightly larger sizer made to complete the rounding operation. Fortunately I didn't end up with any bulges. Now, I didn't really force fit the small sizer, just set it in the horn in a snug postion. At the end of two days I removed the sizers and all seemed well. I grabbed my little hook tool and was able to retrieve the small sizer without incident. By then I was ready to turn the base plug and install.
I want to say I didn't invent this solution. If memory serves me correctly I believe I read about this procedure somewhere. Memory ain't that good so I couldn't say in what publication or just jawing around a camp fire someplace.
Gary