Ok, now I have enough info to give advice...
You built a new support for the tank, but miscalculated by about 1/8", which seems to be the amount by which the plug is now too high. The simplest solution would prob'ly be to correct your mistake so that the tank sits where it belongs.Do NOT try to crank the plug down any tighter...threaded fittings in plastic tanks are NPT standard, which is slighty tapered...so tightening the plug--or any fitting--more than 1.5 turns past hand tight will result in a cracked female "collar." An inspection port isn't likely to sit any lower than your plug, so I don't think it's the solution. Ronco Plastics http://www.ronco-plastics.com has supplied tanks to Catalina for more than 20 years...you might try calling them to see if they have a plug that will sit 1/8" lower. If they don't, I think your only viable solution is to correct your mistake so the tank sits where it was designed to sit.Sometimes we just have to do it over and learn from the experience.