I am not any type of an expert here, but,and this is a big but, by the book the MOT tester is obliged to fail it for that type of repair.
No matter how you dress it up, the book says that any repair should be to manufactured standard or higher, boot floor spot welded in and sealed not bonded, its a weld repair not a bonding exercise.
before you say it is a good enough repair and it is legal, and so and so did it and it's fine, it's not legal and it's not fine, I bought an old 1991 D1 for a grand with 3 months on the ticket and when it re-test time, the previous MOT test station got a ministry visit after my car failed for a sealed in boot floor, do it if you like but I have warned you!!!
Besides you do not have to seam weld it in, just drill it and plug weld it onto good metal it is the same as a spot weld, or you can do inch long beads every 3-4 inches and just underseal it properly, thats what I did when I had to fit a new one after ruining the other when taking it out.