That's indeed not a typical aquarium size, then you say nothing fancy and cheap. But those off standard sizes are usualy only offered by the fancy pancy. So if you find it, it probably will have a fancy pancy price tag.. Or you need to find an aquarium builder, building it for you.. Or just build your own, it aint as difficult as seems and in those small dimensions even less difficult.
I would combine both techniques, use the first video as guide (is the strongest and easiest, bottom panel inside) and apply masking tape as in the second video to prevent smearing kit where you do not need it. Easypeasy to build some fancy pancy..
The glass you need is 4mm blank floated which also is relatively cheap.. The Bison glass silicon kit is perfect for aquarium and available in almost every hardware store.
The regular aquarium industry is for what ever reason still stuck into Dutch style aquarium sizes, higher or same hiegth as width. Only special brands offer off sizes for special prices..
Tho 20 cm high if very often used in the turtle tank, so visiting the reptile/amfibian corner you might find some there. But where i come from, those reptile tanks are more expensive than aquariums.