yeah, those metal ones can be expensive but they may be a good solution...
you can clamp the resistors to a metal heatsink of some sort, but as nigel said, using a higher-wattage resistor is generally a better option. Alternately, you can make a larger series-parallel combination to get the wattage you need. (for instance, last year I needed a 200W, 20 ohm resistor, and I had 20 ohm, 50 watt resistors... so I made two pairs of two 20 resistors in series per pair, and then put the pairs in parallel. total resistance of each branch = 40 ohms, then in parallel it goes back down to the desired 20 ohms... total power rating of the combination = 200 watts)
basically you split the power load across multiple resistors so each one is only dealing with a fraction of it, and then none of them should get very hot, especially since your starter shouldn't be running for a very long period of time at once.