Battery Box Corrosion repair

I’ve just removed the passenger seat of the 90 and looked inside the battery compartment and found the first real signs of rust. I want to do a decent job of it but not sure the best way to go? It will involve someone else doing the welding as i’ve never done it and have no kit. Before I approach anyone i’d like to know the best way of getting it sorted?
There is one real hole, the rest seems surface corrosion that i assume I can wire brush, ‘Kurust’ it then paint it. I’ll also work out how to secure the battery correctly as it was just floating around in there!!
Pics below if you can work the orientation out.

I'd take an angle grinder with a wire cup to it. See what you really have left. It will be a prat to get to as access is quite tight. If you think its bad enough or if you want to do an A1 job then weld in a repair patch. Oh not forgetting a battery retention strap, asking for a short or a cracked battery without that. If its only that small split then I'd be half tempted to fiber glass it. As long as you get back to good metal and use a rust converter nobody will ever see it. Once its all good use plenty of stone chip inside and out.

Think, why did it corrode in the first place? Rock damage to the outer? Rubber grommets missing letting water and dirt in? Make sure you sort causes too or you'll be doing the same thing again in a winter or two. If the battery tray is like that then the footwells might be in a similar state so have a poke around.

Will give it a good rub down and see how solid it is underneath. Not sure why the hole would have appeared there, can't see any damage on 1st looks but must be a reason. Not sure about the filler, assumed patch welding was the preferred choice?

A little fiberglass mat and some epoxy would do me for that location. The battery tray front is only really for keeping water out and the battery in. If its in a shocking state then weld is the way. Do the prep yourself if you are getting someone else to weld it. Save your self some pennies!