Unless it is made of that high-impact plastic that some fenders are made of, I'd be very leery about puting anything plastic on the underside of the teardrop if it isn't protected by some kind of metal frame. The "pans" for my footwells and under-floor storage are made of 16 guage sheetmetal, and anytime I put a water tank down there I surround it with an angle-iron framework. I know the likelihood of it ever sustaining serious damage is pretty slim, but debris and road hazards are out there, and I'd hate to have my (or any of my customers') outings negatively affected by the results of such damage.
With regards to the accessibility to under-floor storage, I guess it all depends on what you put in there and how often you'll need to access it. In 23 years of Teardropping, I've never really found a need for it, but as many of you may know, I
do install and
use the dropped footwell and table feature on nearly all of my personal teardrops. I typically travel with it in the up position, and
sometimes that footwell comes in mighty handy for odd things "collected" along the way ...

...
If it is convenient for you, under-floor storage has some advantages over tongue-boxes:
• it puts the added weight below the floor (lower center of gravity) and closer to the axle centerline, thereby reducing the stress (both static and active) on the tongue-to-mainframe joints by reducing or minimizing tongue-weight
• and on most designs, it allows for a cleaner front profile (IMHO)
Bottom line is, if it works for you and your needs, under-floor storage can be a welcome and useful feature ... just be sure you fully consider your choice of materials and the possibility of encountering potentially damaging hazards while in transit.
Good Luck with your project, and in the meantime ...
CHEERS!
Grant