OK it's done. My siliconing wasn't as good as I thought, and it had a small leak in the bottom (the glass wasn't quite cut to size so I had to do quite a lot of filling in). I decided I didn't mind, though - if I take it out, it will be easier to let it drain from the bottom than tipping, especially as I won't empty all the stuff inside it into the tank.
So, problems and what I would have done differently:
1. I have just left the top open as the intake. It only gets stuff which floats to the top - not the stuff which settles on the bottom. There is now enough flow that most stuff does get floated up, but inevitably some settles and stays. I have a piece of glass I could use as a lid, but I need to get some holes cut into it so I can fit tubes through. This won't happen for a while, it will do for now. I've tripled the filtering on the tank, so I don't mind too much - at least my water won't have any ammonia in it

2. I made it 30cm high, which is too much - it takes up quite a lot of the corner it is in. I was constrained by the size of the 'egg crate' that I used - next time I will cut it better!
3. It's really ugly. But cheap, so I don't mind that much!
4. I didn't protect myself properly from the glass/sharp blades/sharp plastic fragments, and ended up with pretty cut up hands and arms. This was due to my own negligence and because all my safety gear is in college and I didn't want to buy more. I'd definitely protect myself better next time.
Good bits - I love knowing that I did it myself, and it saved me a lot of money. Rough price (in GBP - sorry I don't have time to convert, you'll have to do it yourself if you really want to).
Pump £14.99 + £4 p&p
Glass £5 (but I negotiated offcuts and got it free)
Sponge £2
Silicone £4.99
Total £30.98 ($53.00 using todays exchange rate)
I also used ceramic rings, 'egg crate stuff' (I have no idea what this is called) and some filter floss, which I already had. I also already had a craft knife which I used extensively, and wouldn't have managed without it.
