hee hee - when you said "monster" bike trailer, I was expecting that you just bought or made a version of the
super trailer :
The local bike coalition has one of these, which I use when I work for them to transport 16 bike racks, fold up awnings, tables, several rubber maid bins of supplies, and sometimes a bicycle to special events. That thing is a monster.
The super trailer doesn't turn over at speed, even with it top heavy from the racks being on rails about 3.5ft high, so you don't really need to worry about that.
Probably overkill for home use though.
I just got a regular child carrier that had its seats gutted, and I used it to get groceries for the first time today (and just in time too! When I got home the air raid sirens went off and the city was told to stay indoors until further notice!!)
I didn't have too much weight, but I can see how the cloth bottom could eventually be a problem.
I'm making a mental note of your solution for future reference. It looks good.
You might want to find a way to attach a couple of red blinkies to the plastic bin. If you ever fill the bin completely, or have something too long to fit inside, you could put the lid on and use a bungee cargo net to hold stuff down to it.