The firmware for the GL-MT300N is newer than the AR-150, but I am not clear if the if the issue with packages is resolved or not. Good question for the GLI forum.
In any case I have found it easier to use WinSCP to manage much of my config for all my routers. It's an "FTP" like interface for your router and has a CLI tool built in. I also use putty (at the same time).
You need to load two packages. From either CLI paste in the following strings.
opkg install openvpn-openssl luci-app-openvpn
Your config files are in the \etc\config\
Your openvpn keys go in \etc\openvpn\
OpenVPN will be installed, but not configured. I usually rename the \etc\config\openvpn file to openvpn_original as this file has a lot of comments that will get wiped, and is just to hard to read. They are good notes for reference. Use the WinScp tools to create a new blank file called openvpn. Select one of the sections (depends on your need => server or client, TUN or TAP) and copy it to the new blank file, then edit for your needs.
I have my open OpenWrt server and client on TAP, so my config may not be very helpful to you if you are planning on using a service. You should be able to get a linux script form most services, and just reformat it to the OpenWrt format (option [parameter]'[value]')
Regarding WinScp. I make a folder on my loacal machine called [Router name], and the folders under it for config, etc and use WinSCP to download the config so I have a readily accessible copy of the config. I also keep a text file in the folder that is the "[router name] config" with notes on what i did (packages, config, mods). Helps me reinstall or upgrade a lot faster in the future.