Instead of a sight tube or sight glass what if you could manufacture or attach something to the underside of the cap that would allow someone to just pull the cap off when cool to see the fluid level? It could be as simple as a ball chain with two different colored balls
along the chain to indicate a high and low, no? Maybe something more like an oil dipstick with a metal rod that has upper and lower markings would be easier to make...I really have no idea and if this suggestion is stupid I would appreciate no flaming.