I've just had a look and I think I can make it 30l possibly 40l, we already have a Betta in a 20l tank so was going to do the same. I will make the tank as big as I can as I have some room at the back where I can make it bigger - this is why I need it made to size, Eau do you think I can still only have a Betta if it is 30l/40l?
I suppose it depends what size you are talking, if it's just a little 80 litre thing that just needs to a be a certain shape then I would ask if a local glazier has any experience making fish tanks. As Bureni said, they need to use the right glass and sealant.
If it's something larger then I'd get a specialist to do it.
A local glazier made my large tank from 12 mm glass and used a special sealant, its is 8x3x2 and cost £120 delivered .The glass used was large pieces from broken shop windows that were recycled. Any glazier can build one but make sure the glass is thick enough, well braced and that the sealant used is the correct type.