Disc covers! When you are on a budget, who needs a dedicated disc? These are about $100 and are custom built for whatever wheel you are running.
These are about the difference in cost between a 60 mm wheel (which you can use for other things) and a dedicated disc wheel.
The only downside is that if you do want to reclaim your wheel for normal riding, it can be kind of annoying to take the cassette on and off.
These will save a decent chunk of time in a TT and I’d put them in the high bang for buck category.