The following URL covers this to some extent.
If you want to work from the inside of the tank you can try the following ... figure out how much longer your arm needs to be & cobble up an extension, or a tool holder, for a tongue depressor. You'll want it to be quick & easy to change tools, or depressors, but also hold it securely (don't forget to visit your doctor & swipe many tongue depressors).
You'll also want to tinker with it dry first, and get the feel of following the bad corner with your tool. You may find that you need to develop a touch for advancing or retracting your tool holder in your hand as you go along the corner. You'll also want to get yourself a good inspection
mirror if you don't already have one, and a bright flashlight, then get comfortable with quickly aiming, illuminating & viewing where you need to.
You might also try a couple of practice globs of epoxy
in similar corners, the idea being to be sure that you're mixing something that can be spread decently ... use your tool holder & pay particular attention to angles & pressures that will make it “chatter” along the corner, then don't do that.
If you have the ability to do so, weld some scraps of aluminum
together & play with the epoxy
. If you're not set up for welding just about any decent wall thickness above 1/4 can be quickly drilled, tapped & butted together for your purposes, or simply clamped with a c-clamp or into a corner jig, whatever you have handy. In a pinch, your kid's bubblegum will do nicely.
I mentioned welding, not because test scraps need to be welded but because I'd just grab the welder & tack together half a dozen samples in a few minutes. Once tacked, holding your corners together would no longer be an issue. Except for screwing pieces together wall thinkness is more or less irrelevant to your purpose, you're just after a similar surface & corner to practice with.
You might also adapt your toolholder to grab a piece of scotch bright & use it to clean the corner up a bit, but don't forget to rinse the tank really good
when you're done patching it & you might try to suck it clean with a vacuum nozzle after you scrub & before you patch.
After you've prepared your corner, gone through the motions or rehearsed & mastered your reaches, mirror use & epoxy you'll be ready to goop up & go for it.
As my grandaddy used to say, why go half-assed when you can just grab both cheeks & have a fine ol' time ...
Good luck, Troubledour