I've made a few things to save a few pennies. The most recent ones were a pair of mudguard extensions for the knockabout bike as the cheap mudguards are too short:
They're made from aluminium sheet and attached with cable ties. I'm quite pleased with them, especially the front one which keeps almost all the road spray off the bottom bracket area.
There was also a pair of lamp brackets so that I could mount two lights on to the rack of each of my main bikes:
Good for a bit of extra visibility and provides back up in case one fails.