If you can run programs such as
socat on your tunnel server, you could use a
ProxyCommand instead of a port forward:
# Only necessary if it matches the *.mydomain.com wildcard. Useless otherwise.
ProxyCommand ssh tunnelbox.mydomain.com nc %h %p
#ProxyCommand ssh tunnelbox.mydomain.com ncat %h %p
#ProxyCommand ssh tunnelbox.mydomain.com socat stdio tcp:%h:%p
#ProxyCommand ssh tunnelbox.mydomain.com netcat %h %p
Uncomment the apropriate ProxyCommand, depending on what tools the tunnel server has installed.
Note that this will set up and tear down a separate SSH connection for every tunnel established, which means connecting will be slower than via SOCKS. (It won't affect performance after connection, however.)
If your SSH version supports it, you could enable connection multiplexing: