I used to use Cockatrice (https://cockatrice.github.io/) to play with friends online. The interface is a far cry from Wizard's MTG Arena, but you do get to build any deck you want for free.

Edit: It's been a while since I played, but does look like for IP reasons you'll need to actually download the cards separately from the actual program. I'm not sure where to do this, but I'd imagine someone on this subreddit or on their dedicated Cockatrice server can help you https://www.reddit.com/r/Cockatrice/