If it is that important for you to do it, buy a domain and try using Let's Encrypt. However, keep in mind that Roku does not use your local DNS server and probably will confirm the name in the URL matches the name in the CN or SAN field of the certificate. So you will have to publish a DNS entry that the internet based DNS servers that Roku uses can locate.
Or, you can try just going to using unencrypted HTTP.
I have Roksbox Cloud installed on several Rokus (two Ultras, two Threes and a Two). Yesterday (04/18) I turned on the Roku Two and Roksbox Cloud is gone. I've been using it regularly - it's on the TV in our workout area. I used it last on 04/16 and it was fine. I did a channel search and I see Roksbox Dropbox, Google, etc, but no Cloud. The channel is still available on our Ultras and Threes. Does anyone know if they have dropped support for Roksbox Cloud on Roku Twos? Thanks for your help!