I know what you mean, the higher mountains like 3000m+ are restricted with space…
You could build a vertical with different loading coils and switch then between the bands…
Or build something like the ATX1080
Becoming 40m resonant without loadingcoils on a EndFed antenna and a short wire idk if it is even possbile. Maybe if u have a tuner that can tune a really high swr.
Of course monoband is not the problem. A lot of windings and the coil is ready
Check out this dipole:
https://www.qsl.net/g3pto/shortant.html
Saw sometimes some random wire projects with an 4:1 or an 9:1 Balun. It seems that they are working with less wire length. But remember more wire radiates better
Also this OCFD Antenna is 20m overall length…
I’m building something like this for peaks with little space:
Heinz @HB9BCB modified his AX2-Antenna:
73
Julian