Well, simply cut a small square or rectangle windows in your tank plastic cover in front of the regulator gauge (this is the point?), then glue a clear polycarbonate piece to cover the hole from behind... it doesn't have to be water seal as an aquarium, but you can use windows/bathroom clear silicone caulk to seal it. You may even use the silicone caulk itself as glue to stick the polycarbonate piece since windows/bathroom 100% silicone caulk are glueing very strong and water seal at the same time. You may also secure everything together with some small screws/nuts through everything (through polycarbonate part & plastic tank cover) but IMO silicone is strong enough to glue and will last forever!
This is a very easy project (and also a very good idea), you may find some small pieces of polycarbonate scraps in an hardware store because buying a full sheet is VERY expensive (100$+ for a 4 foot sheet).
2016 Dodge Grand Caravan SE Plus
2011 KZ Sportsmen Classic 16BH