Dan,
if you can't get the exact measurements off an existing part the critical measurements would be the diameter of the handles the mounts sit around, the angle of the handles, the distance between the handles at the point the mounts sit, the diameter of the tank at each end which will sit in the mounts and the profile of the end of the tank.
All fairly easy to do from you mower if the handles are mounted on the base. First off we will address the angle, if you can pick two points one top and one bottom on the handles and measure for me the distance apart at these two points, I also need to know the distance apart the two marks are vertically, I can then work out the angle.
The second dimension will be the distance between the handles at the centre line of where the mounts are to go.
The third dimension is the length of the tank outside to outside at the centre line of the tank, if the tank has flat ends that is easy, if it has convex ends sit the tank on a flat surface and with a square sitting on the surface touching the tank mark the surface with a pencil line, without moving the tank do the same at the other end, you can then measure between the lines on the flat surface to get the tank length.
Now the trickier one is if the tank end is convex I need a copy of its profile, I dont know if you have, or have access to a profile gauge (google multiple needle profile tool) if you can get the profile of the tank and draw it on a piece of paper like a smiling mouth, at each side of the mouth put a mark which will coincide with the diameter of the tank, you then draw a line across the top of the smile from one mark to the other.
The dimension I need from that is the distance from the straight line to the smiley mouth at the centre point ( that woul be from the top straight lip to the bottom curved lip of our smiley mouth.
Dan hope I haven't confused the /// out of you but if I get all that I can then draw up the parts and print them, the reason they were rubber would be to suppress the vibrations on the fuel tank which 3D printed mounts will not do, but if we leave say 4 mm gap between the mounts (2mm each end of the tank) when you mount it all up put silicone in each mount.
This will serve 2 purposes, it will hold the tank in and minimize some of the vibrations.
I have saved a copy of the photo of the mounts from that ebay ad so I know roughly what they look like.
Beech
