which makes using a 50w instead of a 20w even better in this case. you can probably get away with NO heatsink on a 50W resistor if it's only dealing with 20 watts of power or less, saving you the hassle of mounting a heatsink, etc.
however, make sure you can FIT a 50w resistor into your project. they aren't small, by any stretch of the imagination. The ones I've been using at work are a couple inches long, about as big around as your finger, with big mounting lugs, and a bunch of cooling fins.
if you were to build the project in an aluminum (or other metal) enclosure you could simply drill a couple holes and bolt them to it, that should be a more than adequate heatsink since you're only running them at up to 40% of their rated capacity.