People ask fairly often, "How do you install the MGA battery cover seal"? So here's my take on it. This is a seal I bought from Moss Motors in 1986. I can't swear that it is an exact copy of the original piece, but it looked right at the time. The molded rubber piece is quite soft, and the cross section dimensions are as close as I can measure. The battery cover has generous overhang in front of the toe board, enough to insert your fingers behind the front edge with no carpet or seal in place. This may be intended to accommodate a lot of body assembly alignment tolerances. As such, the cover will sit on top of the seal, probably not intended to seal in front.

It appears that the rubber seal may have been intended to work with or without carpet on the heel board. In schematic it looks best installed with the thick edge on top and the longer edge down, as shown above center. Many people report good success gluing the seal in place on clean painted plywood, although staples may work as well (especially on older or dirty plywood). If you install the seal before carpeting, you might truncate the carpet at bottom edge of the seal. There might be space to lay carpet over the seal right to top of the toe board, but you should check fit of the battery cover before to try that.

In my case I glued carpet in place up to top of the toe board before installing the seal, as shown above right. Then I placed the longer edge of the seal on top with thicker edge in front and stapled it to the top edge of the plywood. After 21 years (214,000 miles) in service I pulled the staples to remove the seal, then reinstalled the same seal after replacing the carpet.

The seal is soft and will conform to the battery cover position, if the toe board is high enough. After 50 years of service most cars have the floorboards replaced, some battery covers may be deformed, and some body restoration work may relocate the battery cover slightly. The final configuration may not necessarily be exactly as original (and original may not have been in perfect alignment). In photos below you can see dust on the carpet in area of the outboard angle of the toe board, indicating not perfect seal in that area. My battery cover sits on top of carpet at the ends, which may not be original. Thickness of carpet there raises the cover ends slightly, perhaps as much as 1/8-inch after compression. In this case it may be prudent to install some foam rubber or other backing strip between the seal and top of the toe board to raise the seal to be better aligned with the cover. It is likely not a big chore to modify this installation after the fact (although staples may be easier to remove than glue).

Addendum, October 21, 2019:
Or, we have this picture from the MGA production line. This has the thinner leg of the seal on the rear side of the floorboard, and the thicker rounded leg of the seal on top.

Even though this seems to be original factory configuration, I would check the fit before attachment. From careful measurements on my car, this appears to leave an air gap without sealing.