I used a balboa tank as a toptank for almost a decade and it was never an issue, like at all. Like worst case I could envision, you could need to tip it to the side as a "reserve" but like, it's the last 1/8 gallon maybe that you're missing. I wouldn't worry about it
but if you wanted to, I'd just braze or weld in any barb. In fact the one time I did add a spigot I just welded in a barrel adjuster, cuz it's a hollow metal tube. that was easy.
I've also had good luck sealing stuff with JB weld including tanks, like big holes, so you could theoretically just glue a metal tube in each side, connect them there and be gtg.