Yes you could do that.
How long have you had the clowns? Have they actually laid eggs before? Clowns exhibit a very distinct behavior when they are about to spawn. Female clowns will not lay eggs until they are at least 2 years old. You will know she is getting ready to lay eggs because her belly will look very swollen. The male will spend most of his time cleaning an area. Typically the eggs will be laid right before the lights go out on the tank. They take anywhere from 6-9 days to hatch.
Paired clowns will show signs of spawning long before they actually spawn. I have a pair in my main tank that have been cleaning and carrying on for at least 9 months but neither is old enough to spawn yet. I also have a pair in my frag system that spawn every 11-13 days. They have been together for over 3 years.
To answer your question, yes you can use a 29g tank for the clowns, however you will need to make certain that the temp, ph, sg
and all other water parameters are near perfect to get the clowns to spawn. If they are still young you may be spinning your wheels at this point though.
Go to amazon.com and look for Joyce D. Wilkerson, she has a book called Clownfishes, buy it and read it. Great information in there about clownfish and breeding them.