Don't feel bad,these things get extremely confusing sometimes
Do the boxes have any portion which isn't tapered that you could easily have a hamster accessing?In that case,the previously mentioned extra box in between that is flat could work.This doesn't have to be anything too big,it just needs to be large enough for the hamster in question to fit through to get complete access between sides while keeping the bins completely connected due to the tapered sides.
If there is not a place you could easily attach an extra box in between,you could potentially cut the sides completely off.This would not be ideal though and you'd need to figure out some sort of protective material for use underneath the bins,perhaps linoleum,in case of urine dripping out of the bins.That method would be much more difficult though
I can't think of any other methods,so if neither would be an option then you might be better off looking into a storebought cage between 80 cm x 50 cm and 100 cm x 50 cm,or another DIY within that size range.Of course,that size range is more suited to Russians,but Chinese and robos could potentially be happy in something a little smaller(between 60 cm x 30 cm and 70 cm x 40 cm).