I made the plinth boxes for the cage Spank did for the Imp (which everyone says very nice things about). The plinth boxes were made from thick square tubing. I used CAD (cardboard aided design) to notch the box so that the box matched the contour of the sill and floor spreader plates. It's basically the same shape as your solo cup stradding the sill and touching on the floor as well as the top and side of the sill. It took a few tries to get it so that the plinth matches the plates along the top of the sill,on the sill slope and the floor.
The top of the sill box overlaps the top edges of the box. Not sure the box being hollow is worse than the tube landing on a spreader plate welded to thin metal like the floor. A compression force would need to collapse the steel plate on top of a 4" box (while also pulling inwards the welds on the edge of the box) plus also shoving the 4" box through the spreader plates on the sill and floor.
The verticalish welds on the sill/plinth junction are not really in shear because the plinth is also sitting on top of the sill and the floor. Both of those would have to fail before the welds on the side of the sill would be in shear. I would think it would take a lot more fiddling to get the forward leg/cage A pillar notched to fit the sill. Lots of angle of the dangle to deal with there and if you fail, you have to bend up another leg and get that correct again..
When it all went together I thought to myself "my ankles are sooo hosed" but it has been zero issue so far. I made sure there was very little overlap for the plinth box in that corner so there were no sharp edges. If I do get into a wild flip I'm sure my ankles will hit it at some point (along with pretty much everything else in the foot well) but in normal use, the ankle/plinth interference fears were unfounded. And the Imp is not a big car.
Another benefit to the plinth boxes is that it was straightforward to add tubing fore/aft and side/side between the boxes so that the seat is mounted to the cage and not the car.
1990 RX7 "Mazdarita" 1964 Sunbeam Imp (IOE 2013 Sears Pointless) 2002 Jaguar x-type (Winner C-Class 2021 Sears Pointless)
Gone bye-bye
1994 Jaguar XJ12 (Winner C-Class 2013 Sears Pointless) 1980 Rover SD1 (I Got Screwed 2014 Return of Lemonites)