I have a trimetal bike shed, similar in construction to the Asgard one.
First things first: if thieves want it enough they will break into it, whatever it's made from. Second, they are built to a budget and to reinforce all the possible areas of entry it going to make it expensive and heavy and people won't buy it.
I have reinforced mine with wooden beams (40x70mm ish) bolted along appropriate places inside, for example along the edges of doors. I have also sacked off the stock padlock and fittings and attached a thick hasp, with bolts right through the 40mm thick timber behind it. I have fitted a serious padlock (see below) to this, which indicates to thieves that this is going to be more effort than it is worth and suggests to try next door.
Someone has tried to crowbar it over the last 3 years but failed due to the wood behind it.
If you want proper reassurance you can also put a ground anchor inside, and chain the bike to that also, using a proper chain. Depending on how expensive your bike is you could look at getting a sold secure gold rated chain (used for motorbikes) which is thick enough to defeat any bolt cutters. get the padlock for this keyed alike with the one for the box, to save you hassle. This will stay inside the box usually; I had to take mine to ED leisure centre to lock my bike up when I lost the key for my d-lock, got a few funny looks as the chain weighs more than 10kg!