I'd probably get your old crappy piece of thin board out that passes as the floor (which warps when wet), buy a bit of good MDF, trace it out cut it with the jigsaw and get yourself a nice solid floor - $15 maybe. Then can just bolt it there now worries.
The other option is to build the same for the vertical battery partition thingo - that is another common mount point. Again, get some nice MDF, cut to shape, and maybe carpet it(bunnings has cheap and reasonable carpet by the metre, grey or black). Then mount your amp vertically.
Otherwise if you want to get fancy with perspex, etc you can probably either DIY with a bit of work and knowhow, otherwise call in an expert.