I've made my own traps out of all sorts in the past, from ferrero rocher tubs, Ice cream tubs to purpose made acrylic ones - but I've found the following while shopping in Ikea!
I think is ideal - its long and thin providing the perfect shape for a good flow rate through it and is a size that is relatively un-obtrusive.
They are 48cm long - 9cm high - sloped from 10cm at the top to 9cm (higher surface area) they even have 2 key holes in the back to put your suckers into!
They hold approx 4L of water
AND Only cost £2.99!
Item number - 201.205.77
Item is called "Rationell Variere" and is a Spice rack - found in the kitchen items section (with the pan hanging solutions etc)
I've now made a good few of these and think I've got it nailed.
Using a wood drill bit I've drilled a grid of 5mm holes in one end, A piece of cut filter foam covers them to filter the out going water. The holes don't go all the way to the bottom of the trap so that it leaves water in the trap when you do water changes
At the other end there is a single hole for water input - in my case this is via a "cheap" small internal filter that comes with a spray bar...
I've used Fluval suckers (pack of 4) as they fit well into the keyhole holes in the trap. and I've cut down Fluval 3+ sponges for the ends.
Anyway - here are a few pics to get your own ideas flowing
(I sell these as a kit if people are interested)
The small internal filter
10mm hole for spray bar
20x 5mm holes for sponge filtered outlet
Filter close up