When I used to do this, I used a solvent based glue as it melts the acrylic and bonds it together.
Something like Tensol 12 should work… https://www.ebay.co.uk/i/131755687310
Tensol 70 is stronger than Tensol 12, but needs mixing prior to use.
You need this kind of bonding as any other kind won’t have sufficient strength. That 3M tape will not last long, in fact I’d be surprised if it worked at all when used on the end of a sheet.
In particular, with a large upright panel like that, you’ll get a fair amount of lever force working on the joint.
If you use a thicker sheet for your upright section, you’ll get a larger surface area to bond. You will see a commensurate increase in weight though.
You could cut a strip to be fixed to both the horizontal and vertical sheets, and affix it behind so it’s less obvious. You would really increase the surface area of the bond, and hence the strength.
Make sure you prepare the surfaces well too. Wipe down with isopropyl alcohol. You might want to consider keying the surface by sanding lightly, too. (Before cleaning down).