One consequence of cutting the Beetle in half is that a new barrier between the inside and outside is created along the truck bed front wall. The chassis under the truck bed is filled with holes and inaccessible cavities so keeping water out is an important matter. See also sunroof drains.
Of primary concern is the long seam of the front truck bed wall that communicates directly with the rear interior floor/carpet as well as the large side channel opening by the rocker panels.
The side channel slopes down towards the cabin and, if exposed to the weather, would become a catchment for water from the front third of the truck bed undercarriage area. This would result in water draining onto the carpet near the seat belt floor anchors.
Mitigation of this issue is an absolute necessity.
VW engineers never considered the need for significant luggage area water drainage. There are two small drains holes on the left and right — one over the gas tank and the other by the exhaust heat shield — but these would not draw water away from the lower channels near the rocker panels.
One approach would be to add a “wall dam” that extends down from the front part of each side wall.
The “dam” would create a “wet” middle compartment, leaving the right and left sides by the rocker panel “dry”.
An added benefit is that the dam would create a waterproof area for mounting the rear speakers through the front truck bed wall.
A second smaller dam is added protection against water flowing down the side channel slope. Although this area should be “dry” due to the protection of the larger wall dam, this is an insurance policy against any water that does make it into the side channel. When the cavity is filled with foam, the result should be waterproof.
Side cavity treatment
The large side cavity that drains directly into the cabin clearly needs to be addressed. One method is to fill it with 3M foam and then rivet on a top cover. This will prevent water from oozing down inside in the cabin.
Sealing the edges along the truck bed front wall and installing the wall dam are still necessary.
After all the chassis holes are covered (except for the three drain holes — one in the spare tire hold and two towards the front), the entire under-bed surface can be treated with a rubberized undercoating.