I've also thinking about what to do for a bed with t-slots. My first thought was, like you, 20mm aluminium and machine channels into it. Then I saw the price of the piece of aluminium I'd need! It's probably the route I'll end up taking though as it's fairly simple.

The alternative option (which I've not priced up yet) is to get some aluminium profile, lay that on the bed so that it's very close to flat and then take a thin surfacing cut. You can then use the built in t-slots. Not sure how much cheaper that will be in the long run though. If you can get away with using regular or even thin walled profile I suspect it would be cheaper.

I've just about finished buying all the parts for my machine, cheap and simple are not available options