Another issue with MDF is it absorbs water and softens, especially at rougher areas like cut edges and drilled holes. So right at the spots it needs to be strongest, it is weakest when wet. Worse, I expect in the UK wet or at least damp is a perpetual condition, rather like the pacific northwest here in the US. 
I just don't want to see you end up with the whole box just falling right off on some wet day, because the bolt holes softened so much they let it fall off.
If you *have* to use the MDF for some reason, I would at least move the brackets that surround the box toward the center (or add some) so taht they make a complete strap around the box, using the same bolts that hold it onto the bike to secure the ends of the brackets. A continous strip of metal around the box would at least let you retain the box on the bike if the boltholes themselves began to weaken enough that they can't support it anymore.
Even better would be to make the support to the bike not just be thru the top of the box, but rather as a wrap starting from the tube above the ends of the box, down the ends, under/across the bottom, as a continous wrap. This would give support to the box so it is not hanging from the bike, exactly. It'd still go off if the strap broke, but it would be better than it is now.
You could also add quick-removal strapping around the sides over the tube, using box/pallet strapping like I have on my battery box on CrazyBike2. There are a few kinds of fasteners out there for them, so just try to arrive at the various stores that get stuff in pallet form when they are receiving their loads (often weekly), and see if they'll let you take the strapping with you, and be sure to get the fasteners. Some of them use spring clips that are easily reused and would work very well for your purpose. Even if you have to cut the strapping to access the inside of the box, you won't need to do that very often anyway, so just keep extra strapping around to redo it when that happens. It'll be extra security against a mounting failure, *and* it will remove some stress from the actual mounting and the box so that a failure is less likely in the first place.
I just don't want to see you end up with the whole box just falling right off on some wet day, because the bolt holes softened so much they let it fall off.
If you *have* to use the MDF for some reason, I would at least move the brackets that surround the box toward the center (or add some) so taht they make a complete strap around the box, using the same bolts that hold it onto the bike to secure the ends of the brackets. A continous strip of metal around the box would at least let you retain the box on the bike if the boltholes themselves began to weaken enough that they can't support it anymore.
Even better would be to make the support to the bike not just be thru the top of the box, but rather as a wrap starting from the tube above the ends of the box, down the ends, under/across the bottom, as a continous wrap. This would give support to the box so it is not hanging from the bike, exactly. It'd still go off if the strap broke, but it would be better than it is now.
You could also add quick-removal strapping around the sides over the tube, using box/pallet strapping like I have on my battery box on CrazyBike2. There are a few kinds of fasteners out there for them, so just try to arrive at the various stores that get stuff in pallet form when they are receiving their loads (often weekly), and see if they'll let you take the strapping with you, and be sure to get the fasteners. Some of them use spring clips that are easily reused and would work very well for your purpose. Even if you have to cut the strapping to access the inside of the box, you won't need to do that very often anyway, so just keep extra strapping around to redo it when that happens. It'll be extra security against a mounting failure, *and* it will remove some stress from the actual mounting and the box so that a failure is less likely in the first place.
