I remember having a lawn mower once - a Victa, good Aussie brand, that had a fuel
cap with this type of guage intergrated into it.
I guess it probably works in reverse to what would be required in a holding tank guage, but I do remeber that it was stepped as it progressed further away from the cap; ie it got narrower along its length - depth
of the tank.
I think this was to give an indication of 1/4 full, 1/2 full etc.
I am sure a plastics specialist should be able to fabricate something to satisfy.