Buying a laptop cover without the laptop present

I need a laptop cover for my laptop. One of those close-fitting sleeves. The only problem is, my laptop's a weird shape: not only does it have a 16:9 aspect ratio, but it also has an extra-large battery sticking out at the back.

That's not the only problem; the other problem is that when I go shopping for a sleeve this weekend I don't want to have to carry my laptop around with me. So either I guess at the right dimensions, or I make sure I buy big.

... Or... I take the expanded plastic spacer that came with my wife's laptop sleeve, and cut it to size! The bits that I cut off, I can glue on the top to increase the depth (it's a chunky Dell beast, you see.) and if I arrange the bits really carefully, like this:

Magical DIY laptop spacer-cum-laptray

... then the resulting assemblage of plastic foam can be carried with minimum heaving around Oxford city centre looking at laptop bags; but afterwards it can also double as a makeshift laptop tray, for use when sitting on the sofa or in bed! The space underneath means the fans should be kept fairly clear, to do their whooshing thing when they need to.

I've looked at a few laptop trays and none of them seem amazing: I could really do with that clearance underneath. But to do that properly, you have to have a tray that's built especially for the footprint of your laptop. Well, now I have that.

What I need next is a sleeve.