I would recommend building and hosting your own custom captive portal. You can do this for free and host it on GitHub Pages. Rather than read the documentation.meraki.com material, I would recommend developers.meraki.com for more information. Here are some example guides, step-by-step to build your own fully HTML capable splash page:
Open Source projects by Meraki:
More code examples by Meraki on Code Exchange:
If you and your friend are new to this, I'd strongly recommend the Captive Portal Learning Lab:
wireless engineer and startup founder, formerly known as "the API guy", now I run a Furapi
, the therapy dog service, and Lowenberg Labs
, an IT consulting company.