I have this small Odroid-U3 board hooked to a 2 TB USB HDD that every once in a while is also used to download torrents.
Since quite often I manage it with a device connected to the LAN via wi-fi I am definitely a bit more confortable if the web interface of transmission is encrypted.
Like many other times nginx come in our help.
– Arch Linux ARM is the OS used –
First of all, edit the following two lines of transmission config file:

[root@arch ~]# vi /var/lib/transmission/.config/transmission-daemon/settings.json

"rpc-bind-address": "127.0.0.1",
"rpc-enabled": true,

Edit nginx config file as follow (assuming the default port used by Transmission WEB GUI is 9091 and hostname/transmission is the url we want to be bind to the WEB GUI):

server {
    listen 80;
    server_name localhost;

    access_log      logs/access.log;
    error_log       logs/error.log;

    location /transmission {
        rewrite ^(.*) https://localhost/transmission$1 permanent;
    }

    error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   /usr/share/nginx/html;
        }

}


server {
    listen 443;
    ssl on;
    server_name localhost;

    access_log      logs/ssl-access.log;
    error_log       logs/ssl-error.log;

    ssl_certificate         ssl/server.crt;
    ssl_certificate_key     ssl/server.key;

    ssl_protocols        SSLv3 TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers RC4:HIGH:!aNULL:!MD5;
    ssl_prefer_server_ciphers on;
    keepalive_timeout    60;
    ssl_session_cache    shared:SSL:10m;
    ssl_session_timeout  10m;

    location /transmission {
        proxy_set_header    X-Real-IP  $remote_addr;
        proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header    Host $http_host;
        proxy_redirect      off;
        proxy_pass  http://localhost:9091/transmission;
    }

}

To create the needed certificates take a look at this post: SSL nginx
This is it.