The tank has some complex curves, but with flat portions that you could measure with reasonable accuracy. In my experience, the major dimensions are usually chosen to be integers, with the free-ranging dimensions of subordinate parts being arbitrarily chosen as integer multiples of 1/16 (in Imperial) or 1/100 (in Metric). There are, of course, exceptions.
The problem is, of course, getting accurate measurements of those subordinate parts.
The first thing I would do is search for existing 3D models of your gas tank. While seemingly unlikely, it's the fastest way to move forward if one already exists.
The second thing I would consider is a slip-over design that doesn't require precise measurements. Maybe something that fits to the top of the tank and is secured somewhere on the frame below the tank.
The third thing to consider is renting a 3D Scanner (or hiring a service). I can tell you that on-site scanning services here in Los Angeles are expensive ($700 for a project I considered earlier this year), but they at least include cleanup of the scanned data and delivery of a solid model instead of just STL.
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