I've done the measurement. The battery is 105cm wide (or so - I'm not going under again to measure to the mm).
If the bolt holes on your cage are further apart than that, you can bolt through. Ours aren't, so we popped to Cheltenham to Cleevely, EV specialists, who dropped out the battery and allowed us to use their lift while we fitted the tank. Overall, the job was five and a half hours. At £100 an hour it cost £550.
Now we know what we're doing, I reckon it would be around two and a half to three hours: 45 mins for them to disconnect and drop the battery, 45 to refit and the remainder would be our fitting time. The biggest time vacuum the first go around was the endless measuring to be sure that the tank would fit and that the bolts and spreader plates wouldn't interfere with the cooling fluid pipes on top of the battery. Here's a pic of a battery to show what I mean:

Handful of points:
I don't care how anyone else fits their tank. All I care about is how we fit ours. There are 382,766 other threads on that subject, all exactly the same.
So its cost you a day off work, at whatever your daily expected rate is . Then £550 to have a system fitted ? Thats cool, all i asked for and probably all anyone else expected was just like asked... the cost. We now know it was £550 .
Further ... i've highlighted it in red.... you now know what you're doing ?

This makes no sense to me as you didn't fit the tank.
Further again :
Scared the boy the other day when I pulled into a gap in a line of 30mph traffic with a touch of vigour the other day. Great fun to drive.
I'll add nothing to that you big macho man you...............

Ultimatley the takeaway i have from this is that its quite an operation to remove a van battery to enable drilling into a floor to secure a water tank. Probably not a job for a home enthusiast.
Vin