M_Boats: Advice Sought: Porta-Potti for '85 M15

Quick question, I want to put a porta-potti in my M15, and I have read all the information on the forums and M boat related sites. I understand that the Thetford 135 more or less fits, once coaxed with a soak in some hot water, then weight and wait. My question is, given the expectation that the fit will be tight in the sunken well where the 135 will sit, will I still want the "Marine" version of the 135--which has a secure-down kit--or will the standard one do just fine? I'm not sure there is really much difference in price, and my concern is also that, without actually seeing one of these, would the secure-down kit actually get in the way? Any advice? Thanks, and Happy New Year to all, Neil Dorf Roseville CA M15 1985