It's charity, so don't kill yourself in the process.
I think what you're doing is fine, throw in some sledgehammer against a tire for rounds if you can gather the hammer and tire. Try to swing it for a minute, then rest, work your way up to 2-3 minutes as you would in a regular round. Keep shadow boxing, if you have access to one hit the heavy bag also for rounds.
I only have experience training in Muaythai, but that's what I would do.
Find a hill and sprint as well, I would suggest doing maybe 5-6 dashes as opposed to the commonly prescribed 10. My (un-tested) theory is that if you do less dashes, you may be able to do 2 sprint sessions (instead of one) since you shouldn't be totally spent the next day, but don't sprint the very next day! still take 2-3 days in between.
You can also do other conditioning/HIIT drills, you basically want to be able to constantly move and be active like you would through the rounds so I'd devote most of your time to actually doing that. If you have focus mitts get someone to do a few rounds with you.